Provide compatibility with Feign

[kdavisk6]

Feignx replaces a number of core Feign interfaces. This non-backward compatible breaking change will hinder adoption for those that have built their own extensions. However, most of the breaking changes are in name only, not concept:

• Encoder => RequestEncoder
• Decoder => ResponseDecoder
• ErrorDecoder => ErrorHandler

This should allow us to create a compatibility library that bridges the Feign components with the new Feignx components, allowing the library of existing extensions to take advantages of Feignx's improvements.