tools/toolbox: Add toolbox-sysext-install.sh script

tools/toolbox: Add toolbox-sysext-install.sh script

The new command will take an extension directory set up
with `meson-build --sysext` inside a toolbox, and turn
it into a merged system extension on the host.

Namely it will
 1. copy the extension directory from the container to the host
 2. set up the extension-release file expected by systemd-sysext
 3. compile gsettings schemas if necessary (using the merged
    schemas directory)
 4. install the extension to either runtime or system
 5. merge the extension

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3498>