Add support for IAsyncEnumerable<T>

Is your feature request related to a problem? Please describe.

The API for enumerating query results isn’t great, because it forces the user to be aware of the fact that the results are fetched page by page:

var iterator = ...
while (iterator.HasMoreResults)
{
    var page = await iterator.ReadNextAsync();
    foreach (var item in page)
    {
        ...
    }
}

Describe the solution you’d like

The FeedIterator<T> API is useful, because it gives you more information (e.g. request charge), but in most cases the user just wants the results, so it would be nice to have a higher level abstraction. The IAsyncEnumerable<T> interface seems perfect for this.

There could be an AsAsyncEnumerable extension method like this:

public static async IAsyncEnumerable<T> AsAsyncEnumerable<T>(
    this FeedIterator<T> iterator,
    [EnumeratorCancellation] CancellationToken cancellationToken = default)
{
    while (iterator.HasMoreResults)
    {
        var page = await iterator.ReadNextAsync(cancellationToken);
        foreach (var item in page)
        {
            cancellationToken.ThrowIfCancellationRequested();
            yield return item;
        }
    }
}

The consuming code would be simplified to this:

var iterator = ...
await foreach (var item in iterator.AsAsyncEnumerable())
{
    ...
}

Describe alternatives you’ve considered N/A

Additional context

The await foreach feature was introduced in C# 8, so this would only be useful to people already using C# 8. However, the Microsoft.Bcl.AsyncInterfaces package is compatible with netstandard2.0 and net461, so it wouldn’t require running .NET Core 3.

[ealsur]

We are working on this at the moment 😄

[paulirwin]

Is there an anticipated release date for the v4 SDK? It looks like the last preview came out almost a year and a half ago. Thanks for any updates!