feat(waku2-lightpush): add waku v2 lightpush protocol v2beta1 wire format

[LNSD]

This PR aims to add to the repository the 19/WAKU2-LIGHTPUSH protocol wire format protobuf files.

• [x] Add the current protocol wire format (2.0.0-beta1).
• [x] Add the optional specifiers to the appropriate fields.