Add Logger Support

[kdavisk6]

The current implementation is locked to SLF4J. We replace this with support for the Logger abstraction.

[kptfh]

Do you mean feign.Logger?

[kdavisk6]

Yes.

[kptfh]

'feign.Logger' is not designed for reactive work. Can introduce ReactorLogger but don't have enough reason for this as SLF4J is also a high level abstraction.

[kdavisk6]

I disagree, and we shouldn't ask users of OpenFeign to throw away concepts that are prevalent across all of our other integrations. There are ways to take a synchronous process and make it reactive. Take the reactor documentation itself, B.1. How do I wrap a synchronous, blocking call?.

Feign favor Composition over Inheritance. It is reasonable for us to be able add support for both a native Reactive approach and an approach that allows users to use existing Synchronous Feign constructs. By doing so we can increase the likely hood of this projects adoption.