Osng 487

This pull request contains the (undocumentet) interface for getting all extension-meta-informations from the osiam server. This interface is needed by addon-administration!
this secured endpoint must be verified in the [ControllerIT](https://github.com/osiam/connector4java-integration-tests/blob/master/src/test/groovy/org/osiam/test/integration/ControllerIT.groovy)