This diff introduces a canBePinned boolean to each of the message specs, isolating the 'source of truth' to the message specs. The next diff will have a function that will be able to read each of these, alongside unit tests and updating appropriate callsites.
Only composable message types can be pinned
Resolves https://linear.app/comm/issue/ENG-5213/introduce-a-canbepinned-boolean-to-message-specs
Depends on D9331