MODE-1712: Browse external content via JCR API (4.0)

This pull request creates a dedicated workspace associated with each configured external source. The name of the external source is used as name for workspace. User is able to access workspace via session and browse the content exposed by external system. 

The new, "external" document store implementation utilizes connectors to communicate with external system.
we already have a `MockConnector` class & test. Can't that be used for unit testing the external workspace navigation ? If yes, it should be used. Otherwise, this might need a "better" name.