APP-6752: Add 'fragmentContents' and deprecate 'old' in FragmentHistoryEvent

[ohEmily]

APP-6752

We do not want docs in the fragment_history collection to include metadata that is shared with the doc in the fragment collection. These duplicated fields are creating a bug in the UI and allowing rollbacks to versions on no-op changes made to immutable fields.

[ohEmily]

iteration note to self: make up-backend chokes on these changes as follows:

data/data.go:2305:21: cannot use *old (variable of type Fragment) as FragmentContents value in struct literal
data/models.go:1076:13: cannot use oldFragmentContentsProto (variable of type *structpb.Struct) as *"go.viam.com/api/app/v1".Fragment value in struct literal
data/test_helpers.go:278:40: unknown field Fragment in struct literal of type Fragment
data/test_helpers.go:828:3: unknown field Fragment in struct literal of type Fragment
make[1]: *** [up-app-local-db] Error 1
make[1]: *** Waiting for unfinished jobs....

edit: going to be a bit less ambitious