Introduce Metadata field in ICS20 Packet data proto

[AdityaSripal]

Introduce Metadata field in ICS20 Packet Data proto. This should be done with new field number so that it is backwards compatible

[alizahidraja]

I can take this up

[alizahidraja]

Lmk the versions for protoc & protoc-gen-go to generate updated pb.go

[colin-axner]

Some context:

Typically changing the packet data changes the packet commitment hash. This is actually not the primary concern of this change since the receiving chain will verify that the data bytes provided by the relayer were committed to by the sending chain. The concern is the receiving chains ability to decode the packet data. In this situation, a receiving chain without the memo field in the ICS20 packet data will fail to unmarshal the packet data and return an acknowledgement error. If the sending chain does not support the new field, it will also return an error if a user attempts to provide a MsgTransfer with the field included.

Protobuf will not encode fields if they are empty. Thus transfers to chains without the new packet data field can continue to successfully occur so long as the memo field is left empty

[nicolaslara]

is there any reason to call it Memo over Metadata?

[crodriguezvega]

is there any reason to call it Memo over Metadata?

PR is open in spec repo and calls it metadata.