Support defining configuration and stream schemas using Pydantic

Pydantic is the most popular data validation and serialization library for Python at the moment. It is used by the new-ish and increasingly popular web framework FastAPI and by other frameworks wanting to support reliable data validation and serialization (like odmantic for MongoDB and pydantic-sqlalchemy).

Alternatives include mashumaro, marshmallow and dataclasses-json. All of these support only a subset of Pydantic's features (no validation, serialization but no deserialization).

I propose leveraging Pydantic to allow the SDK user to define a plugin's configuration and inline stream schemas using a known, powerful and well-documented library. The implementation would look something like the following:

from pydantic import BaseModel

class BaseSchema(BaseModel):
    # Created by the SDK developers to support the specifics of Singer schemas

class TapTestConfig(BaseSchema):
    # Plugin config created by the SDK user

class ExampleStreamSchema(BaseSchema):
    # Stream schema created by the SDK user

Notes:

• BaseSchema needs to extend pydantic to make sure schemas are Singer-compatible: https://pydantic-docs.helpmanual.io/usage/schema/#schema-customization
• Initial discussion for pydantic use: https://gitlab.com/meltano/singer-sdk/-/issues/10#note_519595958