Add method level callback for HttpRequestValues initialization

[rstoyanchev]

We initialize HttpRequestValues from @HttpExchange annotations, and invoke argument resolvers to have request values added. It would be useful to have an additional callback that takes into account the method and all arguments with the ability to contribute to the HttpRequestValues.

Spring Security can use this to check custom annotations, allow use of SpEL expressions that reference method arguments, and add request attributes. These can in turn be used for conditional processing in a client interceptor as usual.