Modularizing annotation processing

[allenxwang]

Currently annotation processing is tightly coupled in a few classes. This will cause maintenance problem down the road and makes it impossible to plug-in a third party annotation processor.

The proposal is to modularize annotation processing and have a unified interface for annotation processor:

public interface AnnotationProcessor {
    void process(HttpRequestTemplate.Builder templateBuilder, Method method);

    void process(HttpResourceGroup.Builder groupBuilder, Class interfaceClass);
}

Built-in annotation processors will automatically register with Ribbon. For third party annotation processors, we use the JDK ServiceLoader to find the implementations of this interface on current thread's context class loader.

[allenxwang]

Fixed in 2.0-RC6