Interface and implementation for generic operation on models (Ex: Import from provider)

[cgourdin]

Hello,

To implement generic operations like Import from provider, we need an interface to describe general operations method, Connector generator must be updated after consequently to generate default implementation of this interface.
On configuration part, the implementation is a singleton and will be instantiated by the Connector general factory after connector plugins is added on java class path.
This functionality will give import from provider on Model@Runtime server, Mart server will be updated to manage this interface and set reserved url automatically by connectors.

The main objective is for the user to access import from provider on Mart server in a standard way.