Add support for the 'workspaces' attribute to indexes defined in WF

ModeShape's index design and JSON schema support an optional workspaces attribute which represents a regex for the name of the workspaces to which an index definition applies.
This essentially allows an index definition to track only a particular workspace and not be interested in changes coming from other workspaces.

We should make sure this attribute is also available in the WF kit where atm. each index definition applies to all the workspaces.