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/tasks.py

@@ -0,0 +1,88 @@
+import time
+
+from django.tasks import TaskContext, task
+
+
+@task()
+def noop_task(*args, **kwargs):
+    return None
+
+
+@task
+def noop_task_from_bare_decorator(*args, **kwargs):
+    return None
+
+
+@task()
+async def noop_task_async(*args, **kwargs):
+    return None
+
+
+@task()
+def calculate_meaning_of_life():
+    return 42
+
+
+@task()
+def failing_task_value_error():
+    raise ValueError("This Task failed due to ValueError")
+
+
+@task()
+def failing_task_system_exit():
+    raise SystemExit("This Task failed due to SystemExit")
+
+
+@task()
+def failing_task_keyboard_interrupt():
+    raise KeyboardInterrupt("This Task failed due to KeyboardInterrupt")
+
+
+@task()
+def complex_exception():
+    raise ValueError(ValueError("This task failed"))
+
+
+@task()
+def complex_return_value():
+    # Return something which isn't JSON serializable nor picklable.
+    return lambda: True
+
+
+@task()
+def exit_task():
+    exit(1)
+
+
+@task(enqueue_on_commit=True)
+def enqueue_on_commit_task():
+    pass
+
+
+@task(enqueue_on_commit=False)
+def never_enqueue_on_commit_task():
+    pass
+
+
+@task()
+def hang():
+    """
+    Do nothing for 5 minutes
+    """
+    time.sleep(300)
+
+
+@task()
+def sleep_for(seconds):
+    time.sleep(seconds)
+
+
+@task(takes_context=True)
+def get_task_id(context):
+    return context.task_result.id
+
+
+@task(takes_context=True)
+def test_context(context, attempt):
+    assert isinstance(context, TaskContext)
+    assert context.attempt == attempt