This PR fixes #1876.

1. Added new constructor to AuthenticatedHttpClientHandler which doesn't automatically set the InnerHandler property
2. Adds extension methods to register the Refit client as keyed service
3. Added unit tests

Why did I add the new constructor to AuthenticatedHttpClientHandler?

• To keep the PR as small as possible by avoiding the duplication of code
• Enable the a behavior compatible with IHttpClientBuilder by ...
  • ... avoiding the usage of obsolete functions
  • ... avoid setting InnerHandler, because this will be done by the builder itself

Future changes (not scope of this PR)

• AuthenticatedHttpClientHandler: Remove the old constructor
• AuthenticatedHttpClientHandler: Remove the innerHandler parameter for the new constructor
• AuthenticatedHttpClientHandler: Fix the call sites
• Don't call the obsoleted ConfigureHttpMessageHandlerBuilder
• Fix the unit tests

Behavioral changes

When no httpClientName was specified, the UniqueName.ForType<T> is used and the string representation of the serviceKey (when not null or empty string) gets appended to the unique type name, e.g. Refit.Implementation.Generated+IFooWithOtherAttribute, Refit.Tests, Version=8.0.0.0, Culture=neutral, PublicKeyToken=null, ServiceKey=keyed