Voltage resting estimate for telemetry

## Feature request

**Is your feature request related to a problem? Please describe.**
No option to use resting estimate for voltage reading in telemetry. This would be a big nice-to-have, so that we're not guessing what the sag compensated battery voltage is when viewing in telemetry, i.e. CRSF telemetry. This also affects "front end" features that are relied upon, i.e. battery alerts in OpenTX controllers.

**Describe the solution you'd like**
Add option to to use resting estimate for voltage reading in telemetry, similar to the current GCS option.

**Describe alternatives you've considered**
Guessing what the sag compensated voltage is.

**Platform**
[ X ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

**Additional context**
N/A

Start by tracing CRSF telemetry voltage handling and the existing GCS option for a resting estimate. The work is done when telemetry offers the requested resting-estimate choice and reports the sag-compensated voltage used by downstream battery alerts.

Written by the indexing model from the issue text.