Support charm versioning

[anastasiamac]

As per https://bugs.launchpad.net/juju-charmstore/+bug/1864903

[vernhart]

To save folks a click, here's the text of that bug:

Currently the charmstore only supports a revision for charms. This can make it difficult to know if a newly available charm is just a patch level update, a minor change, or a major revision.

If charms supported specifying a version, similar to snaps, then the charm developers could supply a version. The revision could increment exactly as before but we'd also have a version that can assist in making upgrade decisions.

I would expect the version would be optional for backwards compatibility. We could recommend the use of semantic versioning in the charm developer docs but, again, it wouldn't be a hard requirement.

[mitechie]

The charms are looking to migrate to a snap store setup which would allow for full track management in the future. When you say "versions" I assume you mean the tracks that can be setup for major versioning?