pw_rpc: Protocol updates for client streams

- Add a CLIENT_STREAM packet for messages in client streams.
- Rename CANCEL_SERVER_STREAM to CANCEL, since it applies to all RPC
  types.
- Remove server responses to CANCEL packets from protocol description.
- Document cancellation of unary RPCs.

Change-Id: I3b15beb2de94f318fac58fc3d682aefe41056676
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48800
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>