sched_ext: Add scx_bpf_events() and scx_read_event() for BPF schedulers

scx_bpf_events() is added to the header files so the BPF scheduler
can use it. Also, scx_read_event() is added to read an event type in a
compatible way.

Signed-off-by: Changwoo Min <changwoo@igalia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>