wgpeerselector: add wgkex support

[lemoer]

As discussed in the gluon meetup today, we want to allow using wgpeerselector in combination with wgkex.

The only (big) missing piece is an HTTPS request to the broker as done here.

I would suggest to add the request here (after the NTP check): https://github.com/freifunk-gluon/packages/blob/308166e3c6b2d571606dd1dbfadd2bb8e31d8f90/net/wgpeerselector/files/usr/bin/wgpeerselector#L359-L375

However, the wgkex broker currently expects requests that include {"domain": "'"$SEGMENT"'", "public_key": "'"$PUBLICKEY"'"} in the http body. From the context of the wgpeerselector this information is not tangible. As of now, the wgpeerselector does not know about "domains", as it does not contain gluon code. So this problem needs to be addressed in some way.

@goligo wants to take a look at this. I am open to discuss this here.