Retrive the Admin prefix from `AdminOptions.AdminUrlPrefix` instead of hardcoding it (OSOE-657) about helpful-libraries

[Piedone]

I believe the idea there is that TypedRoute is not necessarily used from the same app where the route is pointing to. It can e.g. be used in UI test projects too, see TypedRouteUITestContextExtensions. Thus, it can't (always) retrieve the target app's admin prefix. Right, @DAud-IcI?

[sarahelsaig]

Yes, it's like this for the sake of UI testing (here is the extension above mentioned). When I created TypedRoute the UI-Testing-Toolbox still used a separate subprocess to run the webapp, so it was literally not possible to access configuration. I see that in the UITestContext the admin prefix is still hard coded instead of loaded from the webapp being tested, so does that mean this is still not possible?

At any rate, if we want to load the admin prefix from configuration, we must make sure if falls back to the sensible default of "/Admin" gracefully and doesn't throw.

[Piedone]

Indeed, in the UI Testing Toolbox we should access AdminOptions, I created an issue for this here: Lombiq/UI-Testing-Toolbox#294

For the UITT use case we'll need to be able to optionally pass the prefix to `TypedRoute too.

[jtkech]

For info, if AdminOptions is not configured, the default value of AdminUrlPrefix is Admin.