Add IAsyncEnumerable<>.SelectAwait accetpting Func<..,Task<>>

I would like to suggest adding the following methods to `System.Linq.AsyncEnumerable`:

```cs
public static IAsyncEnumerable SelectAwait(
this IAsyncEnumerable source,
Func> selector);

public static IAsyncEnumerable SelectAwait(
this IAsyncEnumerable source,
Func> selector);

public static IAsyncEnumerable SelectAwaitWithCancellation(
this IAsyncEnumerable source,
Func> selector);

public static IAsyncEnumerable SelectAwaitWithCancellation(
this IAsyncEnumerable source,
Func> selector);
```

This would allow using selector functions which already return a `Task`;

I currenly have the following code:
```cs
FileList
.ToAsyncEnumerable()
.SelectAwaitWithCancellation(async (f, ct) => await File.ReadAllTextAsync(f, ct))
.Select(LoadFile);
```

If the requested overloads where added I could write is as:
```cs
FileList
.ToAsyncEnumerable()
.SelectAwaitWithCancellation(File.ReadAllTextAsync)
.Select(LoadFile);
```