Add a "callback interval or next callback" field to tavern.

[hulto]

Is your feature request related to a problem? Please describe.
Right now we aren't able to tell if a beacon has missed a callback/died. Ideally we'd display this clearly in the UI so users can remove or ignore dead beacons.

Describe the solution you'd like
Have imix report its next checkin or current callback interval during checkin or through a separate mutation.

Describe alternatives you've considered
Na

Additional context
Na

[KCarretto]

This will be moved to the next release

[hulto]

Kyle:
"Then just need to add a field and update this code to set it from the proto.
https://github.com/KCarretto/realm/blob/c122a523b01c4d5922270e4d6ab113185fa14b15/tavern/internal/c2/server.go#L69"

[KCarretto]

This is done