Add example for Refit and client credential

Add an example and test that validate client crediantials with refit.

We have done this in our other project and can take a look at these links

Refit
https://github.com/FHIDev/Fhi.Grunndata.OppslagWeb/blob/48907f9acf750f6a8dde6bb358d6ef8b9bbaaa7c/Fhi.Grunndata.OppslagWeb/Extensions/HostingConfigureServicesExtensions.cs#L95

Registration
https://github.com/FHIDev/Fhi.Grunndata.OppslagWeb/blob/48907f9acf750f6a8dde6bb358d6ef8b9bbaaa7c/Fhi.Grunndata.OppslagWeb/Extensions/HostingConfigureServicesExtensions.cs#L232

[jennyhougen]

@bvst I have added a Refit sample #24