Explain Stargate / Grpc query and Stargate / Any message from a contract POV

[chipshort]

Basically, we want to tell people that this uses protobuf for the message / query and response type,
that they should use some protobuf crate for that (we can also reference crates like cosmos-sdk-proto and mention that many chains offer their own crates with predefined types) and show some example code on how to do it.
For messages, there is one example in the IBC docs already that can be adapted.

For Stargate query specifically, we should mention this caveat.