Account Pubkey Encoding

[alexanderbez]

Due to x/auth Protobuf encoding migration via #5533, Account.Pubkey was refactored to be a bech32-encoded string instead of crypto.Pubkey, where we convert when we need to. However, this is problematic when the pubkey is a multisig because bech32 limits the decoding to 90 characters which a multisig can exceed 90 chars (amino encoded).

Until Tendermint allows pubkeys to be just []byte slices, we must either:

1. Revert to crypto.Pubkey and use an adhoc oneof in the SDK.
2. Use Amino and keep the type as a string.

I propose we go with option (2) as its the easiest. Note, this is also only temporary, as the next major release of Tendermint will allow us to use []byte in conjunction with interface_type.

/cc @marbar3778 @zmanian

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[zmanian]

Here is what the Tink(Google's core cryptography library) project is thinking for their protobuf key encodings.

https://github.com/google/tink/blob/master/docs/WIRE-FORMAT.md