Simplify security usage in standalone webclient

[tomas-langer]

Currently when we want to invoke a secure request, we need to:

1. Create security instance
2. Create WebClientSecurity service
3. Register service with client
4. Invoke the client

This is too complicated. It would be nicer to have dedicated services, to be able to do something like this:

WebClient.builder()
  .register(WebClientBasicAuth.create())
 .build();

webClient.get()
 .uri("http...")
 .property(WebClientBasicAuth.USERNAME, "jack")
 .property(WebClientBasicAuth.PASSWORD, "password")
 .request()
 ...

or

webClient.get()
 .uri("http...")
 .register(WebClientBasicAuth.create("jack", "password")
 .request()
 ...

Then we could have a builder to configure details of the service behavior (similar to HttpBasicAuthProvider).

Similar services should be available for other security providers, such as OIDC, HTTP-Singatures.