ICS27: Define De/Serialization format

[ethanfrey]

I read that there is one type EXECUTE_TX, which is fine.

And when received, we should parse the bytes into multiple messages: https://github.com/cosmos/ibc/tree/master/spec/app/ics-027-interchain-accounts#packet-relay

However, the serialization format is never defined in the spec. This make building a compatible implementation impossible. Please define this format explicitly here: https://github.com/cosmos/ibc/tree/master/spec/app/ics-027-interchain-accounts#packet-data

Also, I must admit that seeing go code lightly masquerading as the typescript spec code (just changed the name from func to function otherwise Go syntax) is a bit surprising.

[colin-axner]

Great suggestion, sorry we haven't addressed it yet.

In ibc-go we currently use proto3 json which results in a json string like such:

{"type":"TYPE_EXECUTE_TX","data":"ZGF0YQ==","memo":"memo"}

We could switch to json which would look like (the "type" encoding changes):

{"type":1,"data":"ZGF0YQ==","memo":"memo"}

If we made the switch, we would continue using proto3 json to unmarshal because the standard json library would fail to unmarshal the proto3 json encoding (which may be sent by a chain on an existing version)