refactor: Remove VRF and add SEDA keys interface

[hacheigriega]

Explanation of Changes

This PR replaces the existing (but unused) VRF signer interface with a new interface sedaSigner. The application can call LoadSEDASigner(loadPath string) to attain an object that satisfies the interface. This interface can then be used by various ABCI methods to provide signing services for the validators.

The SEDA keys utility will also serve as a single source of truth regarding the indices and their corresponding cryptographic curves.

This PR also removes the following unused code:

• Remove x/randomness for now (Related issue ✨ Re-enable x/randomness with PKR support #314).
• Remove the custom staking module's create-validator-with-vrf and the corresponding gentx logic.

Related PRs and Issues

Related #314
Closes #364

[Thomasvdam]

It would be nice if we can make the key index a known constant. So the batching module can do something like

import (
  "github.com/sedaprotocol/seda-chain/app/utils"
)

// Assume we already have a SEDASigner
sedaSigner.Sign(input, utils.Secp256k1_index)

I do realise that this is basically a map with extra steps, so maybe that's worth considering?

[hacheigriega]

I do realise that this is basically a map with extra steps, so maybe that's worth considering?

Yeah great point. I did think about it before, but the general phobia of maps held me back. It shouldn’t be an issue here since the map iterations will be done off-chain. I’d appreciate your second look!