arti: Support disabling onion services in the config

If an onion service is added to arti's configuration file, arti will always try to run the onion service. Currently to disable an onion service, you need to remove the service from the configuration. But this also means that the service is no longer available to commands like arti hss --nickname foo onion-address, which will just print out "arti: error: Service foo is not configured".

Users may want to prevent their onion service from running, but may still want to be able to see information about their onion service.

I propose adding a per-hs option such as disable = false or run = true, so that a user can choose not to run the onion service, but still have it supported by other arti cli commands.