docs: add x/fees spec (#833)

## Description

This PR adds the spec folder to the x/fees module, containing the whole specifications.

Depends-On: #834
Depends-On: #842

---

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x/fees/spec/01_concepts.md

@@ -0,0 +1,16 @@
+<!--
+order: 1
+-->
+
+# Concepts 
+
+## Minimum fees
+A minimum fee represents the minimum amount of `sdk.Coins` that must be present inside the transaction fees when broadcasting a message of a given type. Setting a minimum fee of `100token` for a message types means that users will need to pay at least `100token` for each message of that type they broadcast. Trying to broadcast a transaction with multiple message of such types will require the user to pay `n * min_fee` fees or above. 
+
+For each message type, there can only be a single minimum fee amount that needs to be paid at any given time. 
+
+A single minimum fee amount can be made of multiple coin amounts, so that if a minimum fee amount is set to `100tokenA,50tokenB` this means that for each message of such type the user will have to pay at least 100 `tokenA` **and** 50 `tokenB` to make sure the transaction is valid. 
+
+If a transaction contains multiple messages of different kinds, each one having a custom minimum fee amount, the overall transaction fee must be greater or equal to the sum of all the minimum fee amounts. So that if a transaction contains one message which minimum fee amount is `100tokenA` and other one which minimum fee is `100tokenB`, the overall transaction fee will have to be `100tokenA + 100tokenB` or greater.
+
+Failing to provide a transaction fee amount that is not enough to satisfy all the minimum fee requirements will lead to an invalid transaction. 

x/fees/spec/02_state.md

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+<!--
+order: 2
+-->
+
+# State
+The `fees` module does not store any state. Instead, it relies on the `params` module to store the [parameters](04_parameters.md).

x/fees/spec/03_antehandlers.md

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+<!--
+order: 3
+-->
+
+# AnteHandlers
+
+The `fees` module presently has no transaction handlers of its own, but does expose the special `AnteHandler`, used for performing a validity check on a transaction, such that it could be thrown out of the mempool.
+The `AnteHandler` can be seen as a set of decorators that check transactions within the current context, per [ADR 010](https://github.com/cosmos/cosmos-sdk/blob/v0.43.0-alpha1/docs/architecture/adr-010-modular-antehandler.md).
+
+Note that the `AnteHandler` is called on both `CheckTx` and `DeliverTx`, as Tendermint proposers presently have the ability to include in their proposed block transactions which fail `CheckTx`.
+
+## Decorators
+
+The fees module provides the following `AnteDecorator`:
+
+* `MinFeeDecorator`: Checks if the `tx` fee is greater or equal to the minimum fee amount computed looking the messages present inside the transaction itself.

x/fees/spec/04_parameters.md

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+<!--
+order: 4
+-->
+
+# Parameters
+
+The fees module contains the following parameters:
+
+| Key | Type | Example |
+|---------|----------|-----------------------------------------------------------------------------------------------------------------|
+| MinFees | []MinFee | `[{ "message_type": "/desmos.profiles.v2.SaveProfile", "amount": [ { "amount": "100", "denom": "tokenA" } ] }]` |
+
+## MinFees 
+The `MinFees` is an array of `MinFee` object, each one made of two different fields: 
+
+* `MessageType` (string), representing the type url of the message which the min fees refer to 
+* `Amount` (`[]Coin`), representing the amount of min fees associated with the message
+
+Inside the `MinFees`, there can only be a single entry for each `MessageType`.

x/fees/spec/05_client.md

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+<!--
+order: 5
+-->
+
+# Client 
+
+## CLI
+
+A user can query the `fees` module using the CLI. 
+
+### Query
+
+The `query` commands allow users to query the `fees` state.
+
+```
+desmos query fees --help
+```
+
+#### parameters
+The `parameters` command allows users to get hte currently set parameters.
+
+```bash
+desmos query fees parameters [flags]
+```
+
+Example:
+```bash
+desmos query fees parameters
+```
+
+Example Output:
+```yaml
+params:
+ min_fees:
+ - message_type: /desmos.profiles.v2.SaveProfile
+ amount: 
+ - amount: "100"
+ denom: "stake"
+```
+
+## gRPC 
+A user can query the `fees` module gRPC endpoints. 
+
+### Params
+The `Params` endpoint allows users to query the current params of the `fees` module.
+
+```bash
+desmos.fees.v1.Query/Params
+```
+
+Example:
+```bash
+grpcurl localhost:9090 desmos.fees.v1.Query/Params
+```
+
+Example Output: 
+```json
+{
+ "params": {
+ "min_fees": [
+ {
+ "message_type": "/desmos.profiles.v2.SaveProfile",
+ "amount": [
+ {
+ "amount": "100",
+ "denom": "stake"
+ }
+ ]
+ }
+ ]
+ }
+}
+```
+
+## REST 
+A user can query the `fees` module using REST endpoints.
+
+### Params
+The `params` endpoint allows users to query for the module parameters.
+
+```bash
+/desmos/fees/v1/params
+```
+
+Example: 
+```bash
+curl localhost:1317/desmos/fees/v1/params
+```
+
+Example Output:
+```json
+{
+ "params": {
+ "min_fees": [
+ {
+ "message_type": "/desmos.profiles.v2.SaveProfile",
+ "amount": [
+ {
+ "amount": "100",
+ "denom": "stake"
+ }
+ ]
+ }
+ ]
+ }
+}
+```

x/fees/spec/README.md

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+<!--
+order: 0
+title: Fees Overview
+parent:
+ title: "fees"
+-->
+
+# `fees`
+
+## Abstract 
+
+This document specified the fees module of Desmos. 
+
+The module allows on-chain governance to specify a minimum amount of fees that users need to pay when broadcasting specific message types. 
+
+## Contents 
+1. **[Concepts](01_concepts.md)**
+ - [Minimum Fee](01_concepts.md#minimum-fees)
+2. **[State](02_state.md)**
+3. **[AnteHandlers](03_antehandlers.md)**
+4. **[Parameters](04_parameters.md)**
+5. **[Client](05_client.md)**
+ - [CLI](05_client.md#cli)
+ - [gRPC](05_client.md#grpc)
+ - [REST](05_client.md#rest)