Add pub key proto definitions

[aaronc]

ref: #6213, after #6214

This PR provides basic protobuf definitions for public keys. It does not implement any functionality beyond the protobuf definitions. That's for other PR's.

It also adds a "Public Key Encoding" section to ADR 020 to describe the approach.

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[aaronc]

@marbar3778 this is what I have in mind for the basic structure of PublicKey. What do you think?

[codecov]

Codecov Report

Merging #6217 into master will not change coverage.
The diff coverage is n/a.

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  Misses      11046    11046           
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[fedekunze]

How would this work for an external application? Eg: on Ethermint we use ethereum's secp256k1 implementation:

https://github.com/ChainSafe/ethermint/blob/542a25ff827036fe9305b154f23ec86f45f48947/crypto/secp256k1.go#L78-L80

It'd be nice to have a concrete example to ease the migration

[aaronc]

I'm thinking there would be two ways to handle this.

One is we don't define the mapping to crypto.PubKey on this type and instead let that be a parameter for TxDecoder, i.e. func (types.PublicKey) (crypto.PubKey, error).

Or, you could use any_pubkey which would specify the full URL (ex. ethermint.crypto.PubKeySecp256k1).

[tac0turtle]

LGTM, will work on bringing over the multisig

[alexanderbez]

LGTM