updata: Librarize add-update and add-migration

[zmrow]

What I'd like:
I'd like to be able to use the add-update and add-migration functionality from updata in other rust code. If possible, we should pull this functionality out into a library!

Any alternatives you've considered:

• Using updata as a binary isn't an option

[sam-aws]

Since these are operations on the manifest it's probably simplest to move the logic from Updata into the existing update_metadata library, and just have Updata call those functions instead.

[sam-aws]

That would also tie in well with #480