sysext: Add minimal support for optional mutability for extensions

sysext: Add minimal support for optional mutability for extensions

systemd-sysext will check if /var/lib/extensions.mutable/${hierarchy} exists
and use it as an overlayfs upperdir for storing writes. This allows having
mutable hierarchy after merging the extension images.

The implementation is following a proposed update to the Extension Images
specification at https://github.com/uapi-group/specifications/pull/78.