Allow Parametrized Types for FeignClient

Originally mentioned in this chat: https://gitter.im/OpenFeign/feign?at=5fc95092a8af805088fdf356

Observe:

@FeignClient(name = "queueKeeperClient", url = "${queueKeeper.api.url}", configuration = QueueKeeperFeignConfiguration.class)
public interface QueueKeeperClient<M> {

    @PostMapping(path = "messages/raw/{queue}")
    void publish(@PathVariable("queue") String queue,
                 @RequestHeader("correlationId") String correlationId,
                 @RequestHeader("msgId") String msgId,
                 @RequestBody M msgBody);
}

Throws parametrized types are not supported!

Generics is a core concept to an OOP language such as Java.
Why was this feature not considered for this when this is compile time checked and optimized?
I presume that this was done for simplicity and to force developers to avoid over engineering ...

Temporary Solution:

@FeignClient(name = "queueKeeperClient", url = "${queueKeeper.api.url}", configuration = QueueKeeperFeignConfiguration.class)
public interface QueueKeeperClient {

    class QueueMessage {}

    @PostMapping(path = "messages/raw/{queue}")
    void publish(@PathVariable("queue") String queue,
                 @RequestHeader("correlationId") String correlationId,
                 @RequestHeader("msgId") String msgId,
                 @RequestBody QueueMessage msgBody);
}

Object could work too and maybe the best fit in most cases.
This is really a 'tag' class similar to Serializable.

If I have the time I would consider trying to solve this or work with someone else on this...seems like an exciting project..

Start by reproducing the parametrized-types error with the @FeignClient and @PostMapping example in the issue. Trace how the generic @RequestBody type is processed, then verify that a parametrized Feign client is accepted without the reported exception. Done means the example works without requiring a concrete tag class or Object workaround.