Move private code into `internal` package + some renames

[jon-whit]

Description

Make internal code, well, internal.

Tasks

1. Move everything internal/graph, except graph.go to internal/check.
2. Move everything in pkg that shouldn't be in pkg to the internal folder:
   • pkg/errors
   • pkg/health
   • pkg/commands. Rationale: we don't want consumers of OpenFGA using the commands directly. They should rely on the methods in server file only.
   • pkg/storage/storagewrappers. (chore(breaking): make storage wrappers internal #1327)
   • pkg/telemetry
3. Rename TypeDefinitionWriteBackend to AuthorizationModelWriteBackend
4. Change storage.ReadChanges. Today, it accepts a string for a continuation token, and returns a []byte continuation token. This is cumbersome. refactor(!): remove token serialization from datastore #2064

[github-actions]

It appears this issue has been stale for at least 14 days 🗓️. If no action is taken the maintainer team may consider closing the issue. Please reach out if you need feedback or follow up actions from the maintainer team.

[miparnisari]

Updated main description with more tasks

[Divan009]

Hey @miparnisari

If no one is working on this, could you please assign this issue to me?

Also, I was hoping you could provide a bit more context regarding Tasks 3 & 6

• What parts of pkg you'd like to keep public, versus what should be moved to internal? Any specific guidance or rules you'd like to follow here?

• The rationale for changing the continuation token in ReadChanges, do you have any specific format in mind for the token, or should I aim to just make the input/output consistent?