Passthru new frames for RPM, Terrain and Fence status

[yaapu]

Eric this adds new frames to support the latest features of my scripts as found here

https://github.com/yaapu/FrskyTelemetryScript/tree/dev

proposed changes:

• added RPM frame 0x500A for rpm1 and rpm2
• added TERRAIN frame 0x500B for terrain enabled/unhealthy
• added FENCE status bits to frame 0x5001
• added THROTTLE to frame 0x5001

Note: this features have not made it yet into the ardupilot codebase so perhaps we can delay merging until they are

[zs6buj]

Great work as usual. Replied on Ardupilot Discourse