add support for streaming with TaskGroups

### Code of Conduct

- [x] I agree to follow Django's Code of Conduct

### Feature Description

see https://forum.djangoproject.com/t/streamingresponse-driven-by-a-taskgroup/40320/4

I'd like to be able to write code that combines multiple streams of data:
```py
async def news_and_weather(request: HttpRequest) -> StreamingHttpResponse:
async def gen() -> AsyncGenerator[bytes]:
async def push(ws_url: str, tx: MemoryObjectSendStream) -> None:
async with tx, connect_ws(ws_url) as conn:
async for msg in conn:
await tx.send(msg)

async with anyio.create_task_group() as tg:
tx, rx = anyio.create_memory_object_stream[bytes]()
with tx, rx:
tg.start_soon(push, "ws://example.com/news", tx.clone())
tg.start_soon(push, "ws://example.com/weather", tx.clone())
tx.close()
async for msg in rx:
yield msg # yield in async generator!! illegal inside TaskGroup!
return StreamingHttpResponse(gen())
```

### Problem

however this doesn’t work because I’m using a yield inside an async generator that’s not a context manager, and calling aclosing() on that async generator is not sufficient to allow a TaskGroup to cancel itself and catch the cancel error.

```py

from useful_types import SupportsAnext

class AsyncIteratorBytesResource(Protocol):
"""
all the machinery needed to safely run an AsyncGenerator[Bytes]

(for django-stubs) this allows AsyncGenerator[bytes] but is less strict
so would also allow a anyio MemoryObjectRecieveStream[bytes]]
"""

async def __aiter__(self) -> SupportsAnext[bytes]: ...
async def aclose(self) -> object: ...

async def news_and_weather(request: HttpRequest) -> StreamingAcmgrHttpResponse:
@contextlib.asynccontextmanager
async def acmgr_gen() -> AsyncGenerator[AsyncIteratorBytesResource]:
async def push(ws_url: str, tx: MemoryObjectSendStream) -> None:
async with tx, connect_ws(ws_url) as conn:
async for msg in conn:
await tx.send(msg)

async with anyio.create_task_group() as tg:
tx, rx = anyio.create_memory_object_stream[bytes]()
with tx, rx:
tg.start_soon(push, "ws://example.com/news", tx.clone())
tg.start_soon(push, "ws://example.com/weather", tx.clone())
tx.close()
yield rx # yield inside asynccontextmanager, permitted inside TaskGroup

return StreamingAcmgrHttpResponse(acmgr_gen())
```

### Request or proposal

proposal

### Additional Details

_No response_

### Implementation Suggestions

https://github.com/django/django/pull/19364/changes