Rudder

To be able to iterate more quicky in data source functionnalities, we want to make it a plugin.

For that, we need to remove all related code, database schema, and dependencies from Rudder main project, and add missing hooks.

More preciselly:

- add a code hook when a node is accepted (mirroring script hook)
- add a code hook when a policy generation is started, before node properties are retrieve (again miroring script hook)
- add two fields in NodeProperties: "provider", meaning who is the owner of that property, with a default to "rudder" if nothing is specified, and "mode", accepting "read-only" or "read-write" option. Only the owner can modify a "read-only" node property. Update node API to use that.