Add a mechanism to pre-load keys and don't enable the API to add new ones

[awlx]

This is a feature for @AiyionPrime and other communities who have a pre-approved list of keys and don't just want to accept any key.

The idea is, the broker loads a yaml file with a list of keys per domain from disc and allows only incoming nodes with a matching key. The list should be refreshed periodical or even better on-change.

[T0biii]

I tried to add this case:
main...T0biii:wgkex:add-preload
but now i am Stuck
maybe someone can do someting with it to make it work

[AiyionPrime]

I doubt we'll ever use wgkex as whole, but am grateful for the effort nevertheless.
Thanks @awlx and @T0biii.

One thing I'd suggest conceptually is named keys, which will make redacting and working with the list a lot easier.
If one were opposed to the idea, one could keep the directory of single-line keyfiles, which do have a name, and generate the yaml using another helperscript.