Add babylon app config

[KonradStaniec]

Add additional section to app.toml with configuration related to bitcoin.

Why app.toml, not genesis params ?

1. Some things are shared between btc-checkpoint and btc-lightclient modules like for example (powLimit). So it makes sense to take it from common config.
2. We are using powLimit to validate proof of work in checkTx which is stateless. The only way to pass configured value there is through global object which can be initialised at start from some global config. (The same thing applies to magic string used in checkpoints i.e we have BBN and BBT)

For now only putting powLimit and "checkpointTag" in config as those are globals used in checkTx validation, rest of params can be decided/added in subsequent pr-s, or per need basis.

[aakoshh]

Just regarding genesis.json vs app.toml - I thought the first would be stuff that we need consensus over, while the latter is for things that are at the discretion of the node operator.

[aakoshh]

Not "0bbn"?

[aakoshh]

Awesome!

[vitsalis]

I’m not entirely sure whether putting the BTC network that is being used and the checkpoint tag in the configuration file is the correct way. My main thinking is this:
Looking at app.toml , it only contains configuration related to how a node is operated (e.g. ports), but not anything related to consensus.
If anyone puts in a wrong combination of configuration (e.g. BTC mainnet, but BBT tag) then this would not lead to consensus.
On the other hand, if we put such stuff in the genesis (or params which are part of genesis) then a node that just runs our code (using the default genesis) is not going to have any issues. However, this would mean that we would have to add genesis configuration for both the btclightclient and btccheckpoint module which would have to be in sync.

I have thought a little bit about this due to the base header of the btclightclient module. Initially, I was inclined to put this in a configuration file, but then decided against it, since this is an important part of consensus and should be the same for everybody. However, with the base header being in the genesis, it is not as easy to setup different types of test networks. For example, if I need to test babylon using a BTC simnet or testnet, I need to create a custom genesis.json file that contains the base header that I want to use. This is not as simple as modifying a configuration file, but I think it is the better way to go.