Allow to configure the Distribution Resource Provider

SCD maintain its own Resource Provider

https://github.com/apache/sling-org-apache-sling-distribution-core/tree/master/src/main/java/org/apache/sling/distribution/resources

The implementation maps OSGI configurations and services as sling resources.
The implementation is not flexible to allow plugging a custom agent in the resource tree.

The mapping seems to be done currently in enums, for instance

https://github.com/apache/sling-org-apache-sling-distribution-core/blob/master/src/main/java/org/apache/sling/distribution/component/impl/DistributionComponentKind.java

This issue is about making the configuration flexible (OSGI properties) or even configuration-less (figure out the mappings at runtime). As a side effect, the implementation may be simplified.

teofili,simone.tripodi FYI