Populate Interface HTTP Request Clients with Authentication properties

[rwinch]

It would be nice if we could provide a flexible way to populate the Interface HTTP Request Clients with information from the Authentication. For example, consider a request that wants to get the currently logged in users profile:

@GetExchange("/users/{currentUsername}/profile")
UserProfile getCurrentUserProfile();

It would be nice if we could provide a flexible way to map Spring Security's Authentication.getName() to the currentUsername path variable.

A few notes:

• I believe this should probably be possible as a Spring Security extension with the current APIs, but need to investigate it further.
• We would probably want to use SpEL to map the property
• The support would look fairly similar to @CurrentSecurityContext and @AuthenticationPrincipal except that we would not necessarily have an argument to put it on.
• This is also similar to the Spring Security + Spring Data Integration
• I'd like to see this in the 7.0.x timeline, but it is likely to slip to 7.1.x

cc @rstoyanchev