Simple way to introduce new Provider (KNOX-179)

This design is intended to address the following issues:

KNOX-179@jira: Simple way to introduce new provider/servlet filters into the chains

The basic issues is that the development and delivery of code is currently required to add a new service into Knox.
This results in a barrier to entry for integrating new services to Knox.
The design below is intended to introduce a very simple configuration based mechanism to accomplish this.
Naturally this will only cover very simple use-cases.
More advanced requirements will still require the development of a ProviderDeploymentContributor.