Make dependency on MP Config be optional

Currently, MP Config is a required dependency for RESTEasy 4.X. When using this in other environments, it is often preferable not to require MP Config. The specific scenario I am running into is that we transform RESTEasy 4.x to be compatible with EE 9 / RESTful WS 3.0, but there is no version of MP Config that will work with EE 9 because of it's current dependency on CDI 2.0 (part of EE 8).

I propose abstracting out the configuration using the ServiceLoader pattern, but still falling back on MP Config if nobody provides a RESTEasy-specific config implementation.

[The need for a fix in RESTEasy 3.x was discovered in the course of looking at the fix for RESTEasy 4.x. -rsigal@redhat.com ]