IAsyncEnumerable SelectMany implementation is inconvienant for common use cases

The most common use case for a SelectMany query I have encountered is to iterate over synchronous lists inside a model. Since the only implementations in this library provide a selector that expects a IAsyncEnumerable result, in most cases you either need to convert the result to an IAsyncEnumerable or write an extension method to handle this yourself.

Implementations for the following operators would be incredibly useful for alot of projects
```
public static IAsyncEnumerable SelectMany(this IAsyncEnumerable source, Func> selector);
public static IAsyncEnumerable SelectMany(this IAsyncEnumerable source, Func> selector);
```