AP_Periph: add ESC_RATE parameter to RC_OUT peripherals

[tpwrules]

Like other vehicles (which use RC_SPEED), ESC_RATE is used to set the PWM output rate for outputs whose functions are set to MotorN so that ESCs can be driven at a fast speed (400Hz, same default as aerial vehicles) while servos still run at normal speed (50Hz, controlled by OUT_RATE).

Tested the PWM outputs with real devices and a scope. Also tested that PWM and DShot600 still fly well.

[andyp1per]

This will need careful testing of dshot. set_freq() can easily mess with the DMA setup resulting in dshot issues. In particular the order of calls is important

[tpwrules]

How would issues manifest?

[magicrub]

I'm not a fan of the param name. Feels like it needs a "PWM" in it instead of RC

[tpwrules]

It is slightly wonky but I took the name from the other vehicles. If we are changing the name I personally would call it ESC_SPEED to fit with the other parameters on periph.

[tpwrules]

Rebased on top of #27444 . It looks like the call to set_freq probably belongs before SRV_Channels::init as that function makes comments about doing late setup.

In any case, the new version has been tested in flight using both 400Hz Normal PWM and DShot600 with a BLHeli_S ESC and performed great in both cases. I also had a 50Hz servo attached to a different timer on the periph to make sure that functionality still worked. Verified the correct PWM waveforms on the scope too.