Make `Annotation` optional

[guzman-raphael]

For example, this would make it easier to instantiate a Pod and load from store.

Some considerations:

• load_pod
  • w/ name/version -> loads annotation + spec into instance
  • w/ hash -> loads spec only into instance
• save_pod
  • w/ annotation -> saves both annotation + spec
  • w/o annotation -> saves only spec
• delete_pod
  • w/ hash -> delete spec dir + all annotations that refer to it (if annotation dir becomes empty, clean up as well)
  • w/ name/ver -> delete annotation only (if annotation dir becomes empty, clean up as well)
• list_pod -> make name and ver Vec<Option<_>> to account for None on "spec-only" pods