Add WireMockHealthCheck in WireMock.Net.Aspire

[Zguy]

I know the contributing guidelines says to make an issue first to have a discussion, but we can have the discussion here and I don't mind if the effort was wasted.

The point of this change is to make WaitFor() in Aspire more useful with WireMock, since it currently doesn't wait for anything. This can be a problem and create a race condition if the main project starts doing HTTP calls to the wiremock server immediately on startup, and will on some runs get a 404 back.

To fix that I've added a health check that calls /__admin/health and checks the result, as well as checks if mappings using AdminApiMappingBuilder has been submitted to the server.

That last part created a catch-22 problem where the mappings were not submitted until the health check was healthy, but the health check was not healthy until the mappings were submitted.

To avoid this, the WireMockServerLifecycleHook class has been slightly re-arranged, and is now using the AfterEndpointsAllocatedAsync callback rather than the AfterResourcesCreatedAsync callback. Within which a separate Task is created that waits until the server is ready and submits the mappings.

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[StefH]

some comments / questions

[StefH]

@Zguy can you take a look at code comments?