Create and use a MethodDescription Pydantic model

[anatoly-scherbakov]

https://dqrura49d0.execute-api.us-east-1.amazonaws.com/docs auto-docs does not describe the format of /methods API endpoint.

To remedy that, create a MethodDescription Pydantic model with two fields: name: str and label: Method.

Use List[MethodDescription] as response_model argument for @app.get() and return list of MethodDescription instances instead of list of dicts.

Relevant docs: https://fastapi.tiangolo.com/tutorial/response-model/

[RomanZorkin]

resolves #17 accidentally sent few issues in one pull request