[Mvc] WebApplicationFactory should implement IAsyncDisposable

[javiercn]

We currently only implement Dispose and call Wait() on the host StopAsync() within Dispose.

Ideally we want to avoid this and simply implement IAsyncDisposable with IDisposable just being a fallback for it.

The only issue here is that currently the factory can be use as an xUnit fixture out of the box, but unfortunately I believe xUnit doesn't support IAsyncDisposable in addition to IDisposable, since they have their own interface for it (IAsyncLifetime).

This means that for using IAsyncDisposable users will either have to extend the default factory and make it implement IAsyncLifetime or create the factory within the test method and use an await using statement to dispose of it.

[javiercn]

This is a follow up to #29404

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[HEskandari]

@javiercn Any objection if I tackle this issue? Looks like there was PR submitted for it, but was abandoned after the review.

[javiercn]

@HEskandari It's fine if you take over from where the other PR left it.

Please preserve their commits so that we can reflect the contribution in any case.