Drop didkit dependency and replace fetch_did with own equivalent implementation; as didkit upstream package availability is unreliable

Add VerifiablePresentation.verify_compliance() and fix tests. Also fix tests related to gx: prefix missing in integrity hashes.

Fix defaults for credential contexts and types, making them more in line with the TS library to fix canonization discrepancies

Create base model classes that cause any credential models to be properly exported via pydantic's serialization by forcing by_alias and exclude_none to True.

Adjusted the API of from_did_url(did_url) for VCs and VPs so that if you call the method on one of these models, they will return the correctly casted type

Add from_did_url(did_url) and from_credential_json(credential_json) helper methods to CredentialModel and some tests

Add from_did_url(did_url) and find_service(service_id) helper methods to DidDocument, some docs and tests