Add post kinds

[SHAcollision]

Fixes #76

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

[tipogi]

Good clean up of struct constructor usingdefault instead of new(). Good point adding in Post struct the post_type property 👍