[Tunnelbroker] Add message types for existing protobuf messages

Summary:
In the transition to a websocket based protocol, we need some way serialize
the messages which will come into tunnelbroker. This change just translates
the existing protobuf message structure into a rust datastructures.

Later diffs will deal with properly seralizing / deserializing the messages, and
likely changing the structure of the messages to be more ergonomic with websockets.

Depends on D7392

Test Plan:

cd services/tunnelbroker
cargo build

Reviewers: ashoat, varun, bartek, max

Reviewed By: varun

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7400