Add Device-level Subscribed flag

[mikenugent1]

We bring in metadata from many PMUs, but only subscribe to measurements from a subset of those devices. It would be great to have a "Subscribed" flag on the Device itself that would be true if any child measurements are subscribed to. This could also just be a user-enterable flag in the Device metadata.

This flag should also be passed across internal-type Gateway connections so that downstream systems (like openHistorian or lower-environment chained SIEGate connections) would know what devices are subscribed to.

[ritchiecarroll]

This is possible - although it will need a little thought in that subscriptions can be made by (1) setting the subscribed flag, or (2) using filter expressions. For the subscribed flag scenarios the DeviceDetail view could easily pick up the flag from the associated measurements - however, for the filter expressions - this a code level operation, not a database one. As a result, a code level operation would ultimately be responsible for assigning the flag.