Add SendTx rpc and proto message definitions

[damiannolan]

Summary

• Add SendTx rpc and message definitions
• Add empty boilerplate func to controller keeper
• Register codec types

service Msg {
  ...
  rpc SubmitTx(MsgSubmitTx) returns (MsgSubmitTxResponse);
}

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

  string owner                       = 1;
  string connection_id               = 2 [(gogoproto.moretags) = "yaml:\"connection_id\""];
  string timeout                     = 3;
  google.protobuf.Any msg            = 4;
}

message MsgSendTxResponse {}

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