Emit control event when a HS descriptor is generated

It would be useful if a Tor controller could detect when a new hidden service descriptor is generated for a local hidden service. Currently a HS_DESC UPLOADED event is produced when a descriptor is uploaded to the HSDir system. However if the descriptor is not being distributed via the HSDir system (PublishHidServDescriptors == 0) then this event would not be produced.

A new event such as HS_NEW_DESC or a new action on the existing HS_DESC event such HS_DESC CREATED should be added.