Make 02-client messages modular

[colin-axner]

Summary

Reduce 02-client messages into a singular concrete implementation so every client doesn't need to reproduce code.

Problem Definition

While implementing solo machine I basically copy pasted the tendermint client messages. I think all clients would end up doing the same since there isn't much flexibility needed in creating clients, updating clients, and submitting misbehaviour.

Proposal

Move MsgCreateClient, MsgUpdateClient and MsgSubmitMisbehaviour to 02-client types with the following definitions:

message MsgCreateClient {
  string client_id = 1;
  google.protobuf.Any client_state = 2;
  google.protobuf.Any consensus_state = 3;
  bytes signer = 4;
}

message MsgUpdateClient {
  string client_id = 1;
  google.protobuf.Any header = 2;
  bytes signer = 3;
}

message MsgSubmitMisbehaviour {
  string client_id = 1;
  google.protobuf.Any misbehaviour = 2;
  bytes signer = 3;

This will reduce redundant code. The only downside I see is storing the client state and consensus state in MsgCreateClient since their might be some overlap.

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