Nima WebClient Builder Config support

[dalexandrov]

In 3.x we had this:

Config config = Config.create();
WebClient.builder()
         .config(config.get("client"))
         .build();

See the javadoc

[arjav-desai]

FYI, as part of PR #7056, we are adding a method to read config and set value on builder based on that. Though, it may not cover all headers/config possible since some features are not yet available (like proxy and cookies) so filed a follow up issue #7078