sched/ext: Add BPF function to fetch rq

sched/ext: Add BPF function to fetch rq

rq contains many useful fields to implement a custom scheduler. For
example, various clock signals like clock_task and clock_pelt can be
used to track load. It also contains stats in other sched_classes, which
are useful to drive scheduling decisions in ext.

tj: Put the new helper below scx_bpf_task_*() helpers.

Signed-off-by: Hongyan Xia <hongyan.xia2@arm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>