R13 — Implement rmw_get_serialized_message_size

**Value:** currently an UNSUPPORTED stub (`rmw_publisher.cpp:570`), but the RMW already holds the `get_serialized_size` typesupport callback.

**Implementation sketch:** one call to the callback + the 4-byte CDR encapsulation header.

**Effort:** ~1 hour. Good first issue.
