📦 Add an entrypoint-based injector loader

It is meant to eliminate import-based injector discovery, allowing the
injector callables to live in different distribution packages.

src/awx_plugins/interfaces/_temporary_private_injector_api.py

@@ -0,0 +1,62 @@
+"""Shared tools for loading injectors."""
+
+from collections.abc import Callable
+from importlib.metadata import entry_points as _load_matching_entry_points
+from os import PathLike
+from typing import Protocol, cast
+
+
+_INJECTOR_ENTRY_POINT_GROUP_NAME = 'awx_plugins.injector'
+
+
+class _CredentialInput(Protocol):
+    def get_input(
+        self: '_CredentialInput',
+        name: str,
+        default: object = None,
+    ) -> bool | str:
+        """Get an input from this credential.
+
+        :param name: Input name to check.
+        :type name: str
+        :param default: Fallback for a missing input.
+        :type default: object
+        """
+
+    def has_input(self: '_CredentialInput', name: str) -> bool:
+        """Check if an input is present.
+
+        :param name: Input name to check.
+        :type name: str
+        """
+
+
+_InjectorCallableType = Callable[
+    [_CredentialInput, dict[str, str], PathLike[str] | str],
+    None,
+]
+
+
+def load_injector_callable(injector_name: str, /) -> _InjectorCallableType:
+    """Load an injector discovered by name into runtime.
+
+    :param injector_name: Entry-point key name under the
+        "awx_plugins.injector" group in packaging metadata.
+    :type injector_name: str
+    :raises LookupError: When no distribution packages provide the
+        requested entry-point name.
+    :returns: An injector callable.
+    :rtype: _InjectorCallableType
+    """
+    injector_entry_points = _load_matching_entry_points(
+        group=_INJECTOR_ENTRY_POINT_GROUP_NAME,
+        name=injector_name,
+    )
+    if not injector_entry_points:
+        raise LookupError
+
+    injector_ep = injector_entry_points[injector_name]
+    return cast(_InjectorCallableType, injector_ep.load())
+
+
+__all__ = ()  # noqa: WPS410