Add user id model

[SHAcollision]

This PR adds a basic UserID model as a wrapper of a String with capabilities to self-validate valid public keys. It replaces the String type on all of our user_id model fields.

Pre-submission Checklist

For tests to work you need a working neo4j instance with the example dataset in docker/db-migration

• Testing: Implement and pass new tests for all new code, while maintaining existing test suite, cargo test.
• Performance: Ensure new code has relevant performance benchmarks, cargo bench