Extend Jackrabbit to allow customized NodeTypeDefDiff validation in Hippo Repository

The allowed changes to a node type definition is validated by Jackrabbit through its NodeTypeDefDiff validator class.

This validation logic and class is:

• very basic and 'not smart', e.g. only 'simple' changes are allowed, (almost) anything else is flagged as non-trivial (MAJOR) and thereby disallowed
• the validator is constructed via a static factory method and its constructor itself is private: extending or customizing the logic therefore requires patching the original class itself

To enable extended and 'smarter' node type changes validation logic and/or further customization in the future without having to further patch jackrabbit repeatedly for each and every case, the current invocation and implementation class will be 'enhanced' to allow further changes to be implemented by an extended (e.g. custom) NodeTypeDefDiff class instead, provided by the hippo-repository.