pybind/mgr/snap-schedule: add typing annotations

pybind/mgr/snap-schedule: add typing annotations

please note, the behavior of `updates_schedule_db()` is changed so that
it now returns the return value of the decorated function. this change
makes it a real decorator which keeps the signature of the decorated
function unchanged. the reason why we need this change is that mypy does
not allow us to match the parameter pack of a Callable at the time of
writing.

Signed-off-by: Kefu Chai <kchai@redhat.com>