Improve async support on the startup path

[davidfowl]

There are a few places where we lack async support that result in people making blocking call. These are using during application startup.

• IAsyncStartupFilter - Add IAsyncStartup and IAsyncStartupFilter to support async startup classes and filters #5897
• Startup (ConfigureServicesAsync, ConfigureAsync)
• IHostBuilder.Configure*
• IConfiguration initial load (keyvault requires an async call)

[BrennanConroy]

We'll put in Next Sprint Planning for now. When are you expecting this work to be done, 5.0 or later?

Thanks for contacting us.
We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We will evaluate the request when we are planning the work for the next milestone. To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[davidfowl]

6.0

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.