Support SSH keystore with workbench

A workbench user should be able to interact with the VFS using ssh and not providing his credentials every time.

To achieve that the worbench should provide a ssh keystore where the user could upload his ssh public keys (in a similar way as github does). This keystore will provide an authentication mechanism to allow key-based autentication when interacting with the vfs.

Although a default implementation will be provided storing the public keys inside the .security folder, it also will provide an extensible API to allow custom implementations and a mechanism for users to choose the implementation.