sched_ext/scx_qmap: Add an example usage of DSQ iterator

sched_ext/scx_qmap: Add an example usage of DSQ iterator

Implement periodic dumping of the shared DSQ to demonstrate the use of the
newly added DSQ iterator.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Cc: bpf@vger.kernel.org