[$] Sched-ext: enqueue() for sub-schedulers and proxy-execution support

The extensiblescheduler class (sched_ext) allows the installation of custom CPUschedulers as a set of BPF programs. While sched_ext, in its current form,has already led to a lot of interesting scheduler-development work, thesubsystem itself is still undergoing rapid evolution. Among other work,the ability to set up a hierarchy of sub-schedulers is approaching completion, anda longstanding incompatibility with proxyexecution is coming to an end.