Add basic proof_cache_tag

[georgeee]

Add basic proof_cache_tag.
No cache implementation, just "identity".

A step towards Use Proof_cache_tag.t in Ledger_proof.t #16469.

Explain how you tested your changes:

• It's a refactoring that doesn't change any behavior
• It compiles :)

Checklist:

• Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? None

[georgeee]

!ci-build-me

[dkijania]

Just a head's up that in my PR which is based on this i wrapped it into (cache_db, [> Initialization_error of Error.t) Async.Deferred.Result.t. is that ok ?

[georgeee]

Surely ok, also the first argument will be string in your case, not unit.