AP_VideoTX / AP_OSD: Make VTX temperature available for OSD

[menschel]

This patch adds VTX temperature forwarding from AP_Tramp through AP_VideoTX to AP_OSD.

I would appreciate feedback about style and correct use of methods because I merely copied code sections based on assumptions without digging too deep into the macros etc.

[menschel]

OK, I have tested and I can see the OSD element but the temperature keeps at 80°C the whole time.
Has this been tested on AP_Tramp, i.e. does the value saved there is accurate ?

[andyp1per]

Main problem is parameter changes

[menschel]

Added changes to the bottom of each location instead of mixing it inside other items.
Do I really need to open up a new group for my parameter? I assume Index 63 is allowed, yes?

[menschel]

OK, I get it, now I have cross talk with TER_HGT. I'll try with a new Group.

[menschel]

I'm getting the impression, that the referenced temperature in AP_Tramp is a thermal protection threshold and not a measurement.

[menschel]

I'll see that I get a second Tramp VTX to solder into some betaflight setup for comparism.

https://betaflight.com/docs/wiki/guides/current/irc-tramp

Just checked that INAV does not have this feature via the stick OSD.
They threw it out in 2020
iNavFlight/inav@9159431

Betaflight should still have it but they did not have a firmware image for the speedybee f405 wing mini
https://github.com/betaflight/betaflight/blob/master/src/main/cms/cms_menu_vtx_tramp.c#L89C5-L89C17

[menschel]

This PR is void.

I received feedback from ImmersionRC / Orqa FPV that this value is a thermal setpoint, means it's stupid to view on OSD.

I'll do something else in another PR to write that setpoint and provide a setpoint value from ardupilot parameters.