Commander class structure instead of scattered static states

[MaEtUgR]

Describe problem solved by the proposed feature
I'm trying to split out all the preflight/arming logic of the commander into modular classes with the main goal to have them more understandable and callable in a modular way on a regular basis. #13224 was the first step of this but now I'm running against a wall adding member variables to the classes and having the class instances be part of the commander module class since there are so many calls from outside static functions.

One chunk of Commander improvements also listed in #7055 is to have the Commander module as a class holding all the split out subfunctions instead of having all the remaining static states and functions e.g. here https://github.com/PX4/Firmware/blob/214e9c8244bfca72a319694ae222727f658ca6c1/src/modules/commander/Commander.cpp#L112-L270

Describe your preferred solution
I think a really big point that is missing is getting rid of the low priority thread for calibration and a solution moving it to the events was last tracked here: #8706 (older PRs for the same thing #8200 #8630) and after that I think it's a lot easier to achieve this goal step by step because the low priority thread is in my eyes the blocker for refactoring since it's more complicated from a scheduling perspective.

I plan on trying to take this up again and do progress. Can we collect the main helping pointers and concerns here such that it gets easy to take a stab?

FYI @dagar

[MaEtUgR]

Update: @dagar Already opened a pr yesterday to work into that direction: #13613 (Thanks for that!)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Thank you for your contributions.

[bkueng]

All done