Add commit metadata to api response and flag for metadataonly

* The blocks were already passing metadata back but it became unused
  at some point in time. Added this metadata to POST api responses
  as the `committed` top level key.

* Adds a new flag, metadataonly, to skip returning the bulk of the api
  response for bots that dont care. Unsure how to properly document
  this for discoverability.

* Updated the commit metadata returned by the blocks.  Everything was
  prefixed with new-, so stripped that.  It would have been nice to
  return revision-id from everything, but some blocks like topic
  need to return multiple revision ids.  For consistency everything is
  now in the form of:  post-revision-id, topic-revision-id,
  header-revision-id, etc.

Fixes T78746

Bug: T78746
Change-Id: I175587314ad0db01f32a522b3d7c58fe17c8b82d
Merge "Add commit metadata to api response and flag for metadataonly"