Improve Schema importer

The current Studio limitation in managing complex type seems to be tied to an ui limitation in creating the schemas.

=> from my point of view this is a safe limitation

But for developers used to XSD this is really too limiting.
If they can import their XSD schemas via the Registry and can then leverage the n levels nested complx types then this will be ok.

For that we need to have :

• a XSD 2 JSON converter too to easily generate the Studio registry entry : see https://github.com/tiry/XSD2JSON
• extend the current DocumentSchemaReader so that it manage complex types recursion : will commit in a branch