[SPARK-3446] Expose underlying job ids in FutureAction.

FutureAction is the only type exposed through the async APIs, so
for job IDs to be useful they need to be exposed there. The complication
is that some async jobs run more than one job (e.g. takeAsync),
so the exposed ID has to actually be a list of IDs that can actually
change over time. So the interface doesn't look very nice, but...

Change is actually small, I just added a basic test to make sure
it works.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #2337 from vanzin/SPARK-3446 and squashes the following commits:

e166a68 [Marcelo Vanzin] Fix comment.
1fed2bc [Marcelo Vanzin] [SPARK-3446] Expose underlying job ids in FutureAction.