Add serializer support for IAsyncEnumerable

[steveharter]

This is a placeholder to uptake the recent IAsyncEnumerable feature for the serializer.

This includes creating an enumerator class that that implements IAsyncEnumerable and accepts a Stream in its constructor (and others TBD) that contains the actual JSON which can be accessed asynchronously:

public class SerializerAsyncEnumerable<T> : IAsyncEnumerable<T>
{
    public SerializerAsyncEnumerable(Stream stream);
    public SerializerAsyncEnumerable(Stream stream, JsonSerializerOptions options);
...
}

and also likely exposed from the JsonSerializer static class:

public static class JsonSerializer
{
...
    public SerializerAsyncEnumerable<T> DeserializeEnumerableAsync(Stream stream);
    public SerializerAsyncEnumerable<T> DeserializeEnumerableAsync(Stream stream, JsonSerializerOptions options);
...
}

The implementation will leverage the same patterns as in the existing async methods which preserve the unprocessed data from the stream (since the stream just returns raw JSON which won't align with object instances) and the "state" for continuing object deserialization as new JSON is obtained.

[stephentoub]

This includes creating an enumerator class

I'm not understanding this. Why is a new public type needed? e.g. why isn't it:

public IAsyncEnumerable<T> DeserializeEnumerableAsync(Stream stream);

?

[steveharter]

I'm not understanding this. Why is a new public type needed? e.g. why isn't it:

It's flagged as still "API needs work" meaning "needs discussion" :)

If SerializerAsyncEnumerable<T> is public it may be convenient to return that instead of just IAsyncEnumerable<T> especially if there is any interesting state to expose (e.g. options, the Stream?).

[layomia]

Closing as dup of #1570.