This provides a couple of much-needed improvements to rbt attach.

It now supports --attachment-history-id=, which allows for specifying

the ID of a file attachment history object that should own the file

attachment. This enables callers to replace file attachments, if they

know the ID, and to set up diffing.
That's perhaps more useful for automation, so to help with that, the

JSON output has been enhanced to provide a lot more information about

the resulting attachment. This includes:

File attachment ID
File attachment History ID
Revision
Caption
Stored filename
Mimetype
Download URL
Review request URL
Review UI URL

This should be enough for automated tooling to begin operating on the

attachment further.
Along with this, the attached_file key in the JSON output is now

guaranteed to be an absolute path, which is better for automated tools.

Tested rbt attach with the new JSON output, with a combination of

arguments (with/without --caption and --filename).
Tested the payload with errors (invalid review request ID, filename).
Tested attaching with an attachment history ID and getting the new

revision.