Provide a configuration source builder

Current, developers that want to define a configuration source can either:

• Implement a component for org.xwiki.configuration.ConfigurationSource
• Inherits from the internal abstract classes from xwiki-platform-configuration-default with is not a good practice

We should provide a configuration source builder allowing developers to configure a chain of configuration (indicate the class where you want to store it, the levels you want, etc.) and generate ConfigurationSource for you to use. You would call the builder in your typed MyConfiguration component #initialize()

Also this should allow to define some configuration that should never be overridden in subwikis.