Fixed #35859 -- Added background Tasks framework interface.

This work implements what was defined in DEP 14
(https://github.com/django/deps/blob/main/accepted/0014-background-workers.rst).

Thanks to Raphael Gaschignard, Eric Holscher, Ran Benita, Sarah Boyce,
Jacob Walls, and Natalia Bidart for the reviews.

tests/tasks/test_custom_backend.py

@@ -0,0 +1,71 @@
+import logging
+from unittest import mock
+
+from django.tasks import default_task_backend, task_backends
+from django.tasks.backends.base import BaseTaskBackend
+from django.tasks.exceptions import InvalidTask
+from django.test import SimpleTestCase, override_settings
+
+from . import tasks as test_tasks
+
+
+class CustomBackend(BaseTaskBackend):
+    def __init__(self, alias, params):
+        super().__init__(alias, params)
+        self.prefix = self.options.get("prefix", "")
+
+    def enqueue(self, *args, **kwargs):
+        logger = logging.getLogger(__name__)
+        logger.info(f"{self.prefix}Task enqueued.")
+
+
+class CustomBackendNoEnqueue(BaseTaskBackend):
+    pass
+
+
+@override_settings(
+    TASKS={
+        "default": {
+            "BACKEND": f"{CustomBackend.__module__}.{CustomBackend.__qualname__}",
+            "ENQUEUE_ON_COMMIT": False,
+            "OPTIONS": {"prefix": "PREFIX: "},
+        },
+        "no_enqueue": {
+            "BACKEND": f"{CustomBackendNoEnqueue.__module__}."
+            f"{CustomBackendNoEnqueue.__qualname__}",
+        },
+    }
+)
+class CustomBackendTestCase(SimpleTestCase):
+    def test_using_correct_backend(self):
+        self.assertEqual(default_task_backend, task_backends["default"])
+        self.assertIsInstance(task_backends["default"], CustomBackend)
+        self.assertEqual(default_task_backend.alias, "default")
+        self.assertEqual(default_task_backend.options, {"prefix": "PREFIX: "})
+
+    @mock.patch.multiple(CustomBackend, supports_async_task=False)
+    def test_enqueue_async_task_on_non_async_backend(self):
+        with self.assertRaisesMessage(
+            InvalidTask, "Backend does not support async Tasks."
+        ):
+            default_task_backend.validate_task(test_tasks.noop_task_async)
+
+    def test_backend_does_not_support_priority(self):
+        with self.assertRaisesMessage(
+            InvalidTask, "Backend does not support setting priority of tasks."
+        ):
+            test_tasks.noop_task.using(priority=10)
+
+    def test_options(self):
+        with self.assertLogs(__name__, level="INFO") as captured_logs:
+            test_tasks.noop_task.enqueue()
+        self.assertEqual(len(captured_logs.output), 1)
+        self.assertIn("PREFIX: Task enqueued", captured_logs.output[0])
+
+    def test_no_enqueue(self):
+        with self.assertRaisesMessage(
+            TypeError,
+            "Can't instantiate abstract class CustomBackendNoEnqueue "
+            "without an implementation for abstract method 'enqueue'",
+        ):
+            test_tasks.noop_task.using(backend="no_enqueue")