rte: add PSU commands and documentation

[macpijan]

Somehow fixes: https://github.com/Dasharo/osfv-scripts/issues/69

I think in the end we do not need that much logic in this CLI tool, but
the PSU control was definitely missing. We provide a single interface
to control the PSU, no matter what power control (relay, Sonoff) is in
place.

It has been added and exposed to the OSFV via RF libs. The CLI tool
should not know much about the firmware behavior, such as whether
the DUT will power on when PSU is enabled, or not. This is often
configurable, even at runtime, and should be handled in test environment
instead. In many cases, the CLI tool may even ope

[macpijan]

Added extra check to warn that the PSU is OFF when we try to use power button, as that was a common mistake:

osfv_cli rte --rte_ip 192.168.10.188 pwr on
DUT model retrieved from snipeit: MSI PRO Z690-A DDR5
Using rte command is invasive action, checking first if the device is not used...
Asset 45 is already checked out by you
Power supply state: OFF !
If you wanted to power on the DUT, you need to enable power supply first ("pwr psu on"), pushing the power button is not enough!
Powering on...
Since the asset 45 has been checkout manually by you prior running this script, it will NOT be checked in automatically. Please return the device when work is finished.

I am not sure if full automation instead would be desired here.

[macpijan]

#77