Add TaskGroup.start() and TaskStatus for task readiness signaling

[kovan]

Summary

• Add TaskStatus class with started(value=None) method for task readiness signaling
• Add TaskGroup.start(coro_fn, *args) async method that creates a task and waits until it calls task_status.started(value), returning that value to the caller
• The task continues running in the group after signaling readiness

This is part of the proposal to adopt proven anyio/Trio patterns natively into asyncio, building on the CancelScope added in #145178. Tracking issue: #145370.

API

async with asyncio.TaskGroup() as tg:
    # start() waits for the task to signal readiness
    address = await tg.start(serve, host, port)
    print(f"Server listening on {address}")
    # task keeps running in the group after started()

The started task receives a TaskStatus object via the task_status keyword argument:

async def serve(host, port, *, task_status):
    server = await setup_server(host, port)
    task_status.started(server.address)  # signal ready, return value to start()
    await server.serve_forever()          # keep running

Test plan

• test_start_basic — returns value passed to started()
• test_start_none_value — started() with no arg returns None
• test_start_task_continues — task keeps running after started()
• test_start_error_before_started — exception before started() propagates
• test_start_cancelled_before_started — cancellation propagates
• test_start_already_started_error — calling started() twice raises RuntimeError
• test_start_multiple_tasks — multiple start() calls in same group
• test_start_with_name — name= forwarded to create_task
• All existing test_asyncio.test_taskgroups and test_asyncio.test_tasks tests pass

[ZeroIntensity]

See my comment.