Incorporate `mount` and `umount` from `imgmount.py` into `imagecraft` itself

> This is more a question for @upils than for you:

> Would this be worth having inside the imagecraft package, possibly with its own entrypoint? It might be nice in the snap to have `imagecraft.mount` and `imagecraft.umount` commands as separate entrypoints.

_Originally posted by @lengau in https://github.com/canonical/imagecraft/pull/94#discussion_r1965746785_

Start by reviewing the mount and umount implementations in imgmount.py and how imagecraft currently exposes commands. Clarify whether they should be incorporated into the imagecraft package and whether separate snap entrypoints named imagecraft.mount and imagecraft.umount are required. Done means the intended command and packaging scope is agreed and implemented.

Written by the indexing model from the issue text.