Set up subscriber surface

[dbolduc]

Googlers can see: go/cloud-rust:subscriber-api (not approved/final circa 2025-12-01)

• [ ] Add message data type
• [ ] Add handler and ack traits
• [ ] Add Session
  • with a way to create sessions from a futures::Stream<(Message, Handler)>
• [ ] Add streaming pull builder
• [ ] Add client

For the handler and session, we do not need the concrete implementation yet.