HttpAuthenticationFeature should have a callback mechanism to get credentials

Instead of configuring the credentials directly on the HttpAuthenticationFeature you should be able to provide a callback. That way, in pre-emptive mode, the credentials can be obtained by asking the user for them on-demand. The HttpAuthenticationFeature should then cache them for the given authentication realm.

I currently implement this with my own ClientRequestFilter that adds the authorization header from a cache of credentials, and a reflection-based proxy in my client that intercepts the 401 status, populates the credentials cache via a callback (where I can present a GUI), then retries the call. It works, but it is the sort of thing that should be built-in if you ask me.
#### Environment
all
#### Affected Versions
[2.17]