New Feature: Support for variable contexts on URIs

It would be nice if via an annotation at class or method level, there could be a means of specifying an implentation that could supply dynamic values to contexts. Consider the following examples:

@VariableContext(alias = "varCtx", supplier = TheValueSupplier.class)
public class Example {

    @Headers({
        "accept: application/json",
    })
    @RequestLine("GET /api/{varCtx}/someEntity/{id}")
    SomeEntity getById(@Param("id") Long id);

}

As it is obvious, the {varCtx} does not need to be passed explicitely via the method's argument. It would also be nice to give the same annotation precedence if placed over a method while one is marking the enclosing class too.

I think this is very handy for cases where a dynamic switch of APIs is needed e.g. between versions (/api/v1/my/path to /api/v2/my/path) or other circumstances (/api/my/path to /api/session/my/path)

The issue names @VariableContext, @RequestLine, and @Param but no repository files or tests. Start by tracing URI template and context resolution around @RequestLine, then define tests for class-level and method-level suppliers, including method precedence; done means dynamic values are substituted without an explicit parameter.