Add QMP6988 telemetry sensor type

[mokjpn]

What does this PR do?

Adds a new QMP6988 value to TelemetrySensorType.

This is a small protobuf change needed for separate firmware support of the QMP6988 environmental sensor. The corresponding firmware PR uses this enum value to identify QMP6988 explicitly in telemetry.

Related Issue: none

Checklist before merging

• All top level messages commented
• All enum members have unique descriptions

[mokjpn]

Closing this PR because it was based on master, while the firmware work that consumes this enum is tracking the protobufs develop baseline used by current meshtastic/firmware. I opened a replacement PR from a develop-based branch to avoid rolling back the NodeDB split protobuf changes when firmware regenerates its nanopb output.