[Feature] Provide API updates while app is cold starting

*Low Impact*

Currently the Portal UX is completely hung while an app is cold starting for the first time after a deployment. When running on an app service plan this can be very cumbersome as the app appears to be broken when it is in fact operating exactly as expected.

I propose the admin APIs be executed in a separate thread that is not dependent on the host starting up. The host should periodically update the management thread so that the user can see that their app is in the process of starting for the first time.

CSS 119071121002948

Start by tracing how the admin APIs depend on host startup and how cold-start progress is currently represented. Determine whether a separate management path can remain responsive and what periodic status update would indicate that startup is still in progress; done means the Portal UX can receive API updates during the first post-deployment cold start.

Written by the indexing model from the issue text.