add release_notes to post-release amqp msg

Motivation: At Method we have a general notification system that generates emails/etc directly from amqp msgs. Even though you can get the rez pkg from the post-release amqp msg, that requires a custom component (ie amqp consumer) which we would rather deprecate. Some redundancy in the amqp message helps simplify things (in general I have found, not specific to rez).

Note that it may make sense to extend this so that all trivial package attributes (ie no funcs/late-bound/etc) are included in the message.

No files, tests, or entry points are named. Start by locating the producer for the post-release AMQP message and the code that serializes package metadata; determine how release_notes should be represented and whether existing message tests cover it. Done means post-release messages expose release_notes without requiring a custom consumer, with the requested behavior tested.

Written by the indexing model from the issue text.