1.2.0

Conditional auth header, model dump, bump version

Bump package version to 1.2.0. Update async and sync clients to fetch the access token into a local variable and only add the Authorization header when a token is present, preventing headers with empty tokens. Use Pydantic's model_dump(by_alias=True, exclude_none=True) instead of dict() when serializing request bodies. Add an optional app_context field to the OAuth2Token model.