Bka backend vdc

• rename modulator.[cpp|h]->dsmeter.[cpp|h]
• dsmeter: log to dsmeter log channel
• introduce BusMember_backendVdc enum option

https://trello.com/c/hXOyPEps/55-integrate-vdc-in-dss-datamodel

https://redmine.digitalstrom.org/issues/22451

Depends on MRs:

• ds485-stack/dsm-api!103 (merged)

Related MRs:

• dss-websrc/dss-websrc-mainline!288 (merged)