This adds a `services.scx.enable` option to enable sched-ext schedulers.
Requires a kernel with sched-ext enabled (6.12+) or a kernel with the patchset.
requiredKernelConfigs are taken from https://cateee.net/lkddb/web-lkddb/SCHED_CLASS_EXT.html
(cherry picked from commit 3e710e6d15)