feat(config): add CLI arg to specify config file

This allows users to create simple "profiles" via separate `invokeai.yaml` files.

- Remove `InvokeAIAppConfig.set_root()`, it's extraneous
- Remove `InvokeAIAppConfig.merge_from_file()`, it's extraneous
- Add `--config` to the app arg parser, add `InvokeAIAppConfig._config_file`, and consume in the config singleton getter
- `InvokeAIAppConfig.init_file_path` -> `InvokeAIAppConfig.config_file_path`

tests/test_config.py

@@ -170,7 +170,7 @@ def test_set_and_resolve_paths():
     """Test setting root and resolving paths based on it."""
     with TemporaryDirectory() as tmpdir:
         config = InvokeAIAppConfig()
-        config.set_root(Path(tmpdir))
+        config._root = Path(tmpdir)
         assert config.models_path == Path(tmpdir).resolve() / "models"
         assert config.db_path == Path(tmpdir).resolve() / "databases" / "invokeai.db"