add support for streaming with TaskGroups

[Thomas Grainger]

​https://forum.djangoproject.com/t/streamingresponse-driven-by-a-taskgroup/40320/4
​https://github.com/django/new-features/issues/117

### 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())

`

### Implementation Suggestions

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