Version Access endpoint

[sbatchelder]

Although mediastore keeps the history/versions of updated media objects, there is currently no way to access them via the api endpoints.

Create api endpoints such that a user can list all PID versions/marks.

Add to media-get endpoint ability to specify a specific historical version (but default to most recent)

Add to all media metadata the mark/version number of the fetched media object, and perhaps a flag that say if it's the latest or a historical entry. At this point, might as well also include 'original creation time' and 'version creation time'