Extend concurrency helper utilities to work with async iterables for arguments

### Things to check first

- [x] I have searched the existing issues and didn't find my feature already requested there

### Feature description

Just a thought, as I'm not sure this is worth the complexity or API change.

Currently, you can't:
- Pass an async iterable to `amap()`'s `args` parameter
- Spread an async generator to use in `gather()`

### Use case

I find myself often writing code like this:

```python
async with create_task_group() as tg:
async for foo in get_foo():
tg.start_soon(process_foo, foo)
```

At first, this looks like what the concurrency utilities should help with, but I don't think it's currently possible.

You'd need to do this to use `amap()`:

```python
await amap(process_foo, [foo async for foo in get_foo()])
```

Which defeats the point of `get_foo()` being async.

Read the amap() and gather() implementations and their current argument handling first. Check the existing tests for both utilities and define the supported async-iterable behavior before changing the API; done means async iterables can supply amap() arguments and async generators can be consumed by gather() without materializing them.

Scritto dal modello di indicizzazione a partire dal testo della issue.