sysext: provide systemd-{sysext,confext}-sysroot.service services (#41161)

This should pretty much close #38985

The new services are used to activate system and configuration
extensions for the main system from the initrd, this allows to overcome
the limitation that sysext/confext cannot be used to update the
resources which are required in the earliest boot of the system (before
systemd-sysext/systemd-confext start).

To make it possible to disable sysext/confext merging logic,
`systemd.sysext=0`, `systemd.confext=0`, `rd.systemd.sysext=0`,
`rd.systemd.confext=0` kernel cmdline options are introduced.

Closes #38985