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/frontend/install/model_install.py

@@ -513,7 +513,7 @@ def select_and_download_models(opt: Namespace) -> None:
     """Prompt user for install/delete selections and execute."""
     precision = "float32" if opt.full_precision else choose_precision(torch.device(choose_torch_device()))
     # unsure how to avoid a typing complaint in the next line: config.precision is an enumerated Literal
-    config.precision = precision  # type: ignore
+    config.precision = precision
     install_helper = InstallHelper(config, logger)
     installer = install_helper.installer