network: Show DNS6 parameters in details and connection editor

Fixes GNOME/gnome-control-center#1231
7 jobs for show-dns6 in 2 minutes and 19 seconds
latest
Status Job ID Name Coverage
  Build
passed #1121280
build

00:01:27

 
  Test
passed #1121281
test

00:00:52

 
  Manual
manual #1121283
asan allowed to fail manual
asan
manual #1121282
allowed to fail manual
flatpak
manual #1121286
allowed to fail manual
msan
manual #1121284
allowed to fail manual
tsan
manual #1121285
allowed to fail manual
ubsan