Add RegisterAccount rpc and proto message definitions

[damiannolan]

Summary

• Add RegisterAccount rpc and message definitions
• Add empty boilerplate handler to controller keeper
• Register codec types

service Msg {
  rpc RegisterAccount(MsgRegisterAccount) returns (MsgRegisterAccountResponse);
  ...
}

message MsgRegisterAccount {
  option (gogoproto.equal)           = false;
  option (gogoproto.goproto_getters) = false;

  string owner                      = 1;
  string connection_id              = 2 [(gogoproto.moretags) = "yaml:\"connection_id\""];
  string version                    = 3;
}

message MsgRegisterAccountResponse {}

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