Add Ix.NET AverageAsync overloads supporting async selectors

[idg10]

System.Linq.Async always offered versions of the AverageAsync operator that accepted projection selectors. These came in three forms:

1. Func<TSource, TResult>-style selectors
2. Async selectors of form Func<TSource, ValueTask<TResult>>
3. Cancellable selectors of the form Func<TSource, CancellationToken, ValueTask<TResult>>

System.Linq.Async is being deprecated because the .NET class libraries now include most of the IAsyncEnumerable<T> LINQ functionality in System.Linq.AsyncEnumerable. However, certain features are missing in that, include projection-based averaging.

In cases where System.Linq.AsyncEnumerable does not offer equivalent functionality, we're moving features out of System.Linq.Async and into System.Interactive.Async.

However, we missed 2 and 3 here. That happened because those overload have different names: AverageAwaitAsync, and AverageAwaitWithCancellationAsync.) Those naming conventions are out of line with .NET guidelines, so as part of this migration we've been deprecating anything using those old conventions. In many cases, .NET's System.Linq.AsyncEnumerable, adds new methods with the same functionality but with proper naming conventions. In cases where .NET has not replicated our functionality, we have followed suit by deprecating these methods and then defining properly-named overloads in System.Interactive.Async.

Except that in this case we only did the deprecation part: we forgot to add the equivalent properly named overloads in System.Interactive.Async. This fixes that.