Implement ratings xrpc endpoint #5

Should be able to create or remove a rating for a post via xrpc procedures.

Steps to implement:

1. Define methods in ISoapStoneLexiconController
2. Implement methods in SoapStoneLexiconController using database and atproto repositories
3. Implement handler methods that user controller in handlers.ts
4. Connect handler methods to lexicon server in server.ts.

Marking as good first issue since there's an established pattern here with the CRUD operations with posts. Code stack might be a little complicated, might want to refactor later.