Enabling CI lint checks

This is the finishing touch for https://github.com/ably/ably-flutter/issues/278. Changes in this PR:

Introduce lint rules to raise errors on missing returns and required params
Ignore same-package deprecated library members to avoid failures when we deprecate a component of Ably library
Cleanup and sort of anlysis rules with some additional checks included
Temporary silenced close_sink rule for one of push module files ( related to https://github.com/ably/ably-flutter/issues/382)
Updated GitHub action to fail if flutter analyze fails

ably / ably-flutter