network: Add option for connection sharing

Introduce a new IP{4,6} config method to allow sharing the default
network (usually the Internet) through the wired interface.
This is needed because the control-panel is lacking this feature backed
by nm and currently the only way to enable the connection sharing is by
using nm-connection-editor.
9 jobs for shared_connection in 29 minutes and 40 seconds (queued for 128 minutes and 49 seconds)
latest
Status Job ID Name Coverage
  Build
passed #67398
build

00:02:05

 
  Test
passed #67401
armv8

00:12:05

passed #67400
asan

00:01:50

failed #67402
allowed to fail
ppc64le

00:12:41

passed #67399
test

00:00:57

 
  Manual
manual #67405
allowed to fail manual
msan
manual #67403
allowed to fail manual
tsan
manual #67404
allowed to fail manual
ubsan
 
  Deploy
manual #67406
allowed to fail manual
flatpak
 
Name Stage Failure
failed
ppc64le Test
Full log written to /cwd/_build/meson-logs/testlog.txt
FAILED: meson-test
/usr/bin/python3 -u /usr/bin/meson test --no-rebuild --print-errorlogs
ninja: build stopped: subcommand failed.
Uploading artifacts...
./*.log: found 1 matching files
_build/meson-logs/: found 4 matching files
Uploading artifacts to coordinator... ok id=67402 responseStatus=201 Created token=nLL6BjvF
ERROR: Job failed: exit code 1