Provide an API for implementing Configuration Sources

A lot of scaffolding and utility classes already exist to implement configuration sources within XWiki : https://github.com/xwiki/xwiki-platform/tree/stable-14.10.x/xwiki-platform-core/xwiki-platform-configuration/xwiki-platform-configuration-default/src/main/java/org/xwiki/configuration/internal

These are however internal classes, and thus we tend not to use them outside of xwiki-platform. It could be interesting to expose some of these classes as APIs, so that they can be re-used to implement configuration sources for extensions.