Relates to tendermint/tendermint#9197

Depends on #94

In order to better support integrators, we need to version our Protobuf definitions as per Buf's guidelines. The goal here is to have:

• Our Protobuf definitions in the /proto folder in the repo. Versioned protobuf files #495
• Our generated Protobuf Go code in a new /api folder in the repo. Versioned protobuf files #495
• Document how to translate protobuf messages across versions by annotating the .proto files
  • First version (ABCI only ATM) of documenting protobuf version translation #780