Move away from unbounded cache for `lastObservedAt` stat, which was powered by BigCache (unbounded), to Ristretto, a bounded in-memory cache (https://github.com/dgraph-io/ristretto).
This PR is first step towards moving away from unbounded caches in the system, more PRs to follow.
## 📄 Summary
- Instead of relying on JWT for session management, we are adding another token system: opaque. This gives the benefits of expiration and revocation.
- We are now ensuring that emails are regex checked throughout the backend.
- Support has been added for OIDC protocol
