Allow to ignore methods on provided interface

When the Feign annotated interface (or class passed to the builder) contains methods that are not intended to be invoked by a web request (i.e. not annotated with RequestMapping), Feign (actually the BaseContract) throws an exception saying the method is not annotated with HTTP method type.

While this is legitimate, I'd like to request an enhancement to allow the contract to ignore methods on demand (kinda like it ignores static methods), for example via some annotation ("@FeignIgnore"?).

The motivation for this is that the same interface class can be used in different contexts, not just for http based communication. Specifically, we use the same interface class both for Feign and for sending messages to remote services via a queue - if there is a method that should only be invoked through a queue, we'd expect it to not be annotated with RequestMapping, which causes Feign to fail to process the class entirely.

Start with the BaseContract behavior described in the issue and trace how methods without RequestMapping annotations are processed. Consider the requested opt-in ignore mechanism for interfaces shared with queue-based messaging, then verify that ignored methods no longer fail contract parsing while HTTP-mapped methods continue to work.