Add wait_for_shutdown utility

[mhidalgo-bdai]

This patch brings a wait_for_shutdown utility to wait for a given context to shutdown. Useful to block the main thread while callbacks are invoked in the background. Builds on #32.

[bhung-bdai]

Similar question to the other threaded PRs: does this shut down only the work being done on that thread, or does it shut down the executor globally?

[mhidalgo-bdai]

does this shut down only the work being done on that thread, or does it shut down the executor globally?

wait_for_shutdown does not shut down anything, it only performs a blocking wait until shutdown or timeout. As it's blocking, shutdown can only happen from another thread or signal handler. It serves the same purpose as rospy.spin did: just hold until ROS 2 shuts down. It's convenient in the main thread when the executor is spinning in the background.

On a second thought, one could also disable background spinning and spin explicitly in the main thread. I'll add that option too.

[ksharma-bdai]

The wait_fot_shutdown portion looks straightforward and convenient, will approve after #32 is merged and this is rebased.

[ksharma-bdai]

LGTM, essentially a 1:1 breakout of the prior code into a useful helper.