Extensions homogenization

The purpose of this PR is to have a function `init` in extensions in need. Thus, it will be possible statically link extensions with the server and then call the `init` function before the loop server (you can see an example in the sandbox folder).

This function `init` is equivalent to the configuration of extensions in the XML file, but in the form Caml. We could at the end delete the XML configuration file.

If you have criticism, do not hesitate! We have not yet identified any problems with @balat.