make model manager v2 ready for PR review

- Replace legacy model manager service with the v2 manager.

- Update invocations to use new load interface.

- Fixed many but not all type checking errors in the invocations. Most
  were unrelated to model manager

- Updated routes. All the new routes live under the route tag
  `model_manager_v2`. To avoid confusion with the old routes,
  they have the URL prefix `/api/v2/models`. The old routes
  have been de-registered.

- Added a pytest for the loader.

- Updated documentation in contributing/MODEL_MANAGER.md

invokeai/backend/model_manager/search.py

@@ -22,11 +22,12 @@ Example usage:
 
 import os
 from abc import ABC, abstractmethod
+from logging import Logger
 from pathlib import Path
 from typing import Callable, Optional, Set, Union
 
 from pydantic import BaseModel, Field
-from logging import Logger
+
 from invokeai.backend.util.logging import InvokeAILogger
 
 default_logger: Logger = InvokeAILogger.get_logger()