ExternalServiceResource does not work if the parameter value is not set

[davidfowl]

Run the following and it will show up as unknown:

var externalSvc = builder.AddExternalService("svc", builder.AddParameter("url"));

This is because it accesses the parameter's value directly

aspire/src/Aspire.Hosting/ExternalServiceBuilderExtensions.cs

Line 107 in da81496

var url = resource.UrlParameter?.Value;

Which throws if it wasn't provided (or can even throw if there's an error getting the value). We should use IValueProvider.GetValueAsync and we should gracefully propagate errors from the parameter resolution to this resource's state.

cc @DamianEdwards