Add list of PV names in the manager

Currently the service manager can check whether the value of a PV can be read or not when testing the PV connection upon adding a new config entry or pressing the test button in the ConfigEntry dialog.
However, there is no list of available PV names, so the users need to get the names from somewhere else.

There should be a simple list containing all HLM PV names and descriptions (and anything else of value) available in the ServiceManager.

Note: The PV Import service is running on NDAHEMON at the moment, which has no connection to the IOC DB (a possible source of HLM PV names, although we only need the HLM PVs and nothing else).

Start with the ServiceManager and ConfigEntry dialog flow described in the issue, then inspect how the PV Import service obtains or checks PV names. Determine whether the IOC DB or another source can provide only HLM PVs while running on NDAHEMON; done means the ServiceManager presents a usable list of HLM PV names and descriptions.

Written by the indexing model from the issue text.