feat: Improve action unadvertising (backport #1248)

[mergify]

Public API Changes
None

Description
Previously, action servers were never actually destroyed and just left hanging. This PR improves the logic so that now, when an action gets unadvertised:

1. Any new goals are rejected.
2. Pending goals are aborted
3. When the last goal finishes, action server destruction is scheduled 1 second into the future to allow all action results to be sent.

The test confirms that the user can advertise the action, unadvertise the action and then advertise the action with the same name again.

I'm still not sure whether action_server.destroy() is perfectly safe but I didn't experience any crashes when testing it.

---

This is an automatic backport of pull request #1248 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   rosbridge_library/src/rosbridge_library/capabilities/send_action_goal.py
	modified:   rosbridge_library/test/capabilities/test_action_capabilities.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   rosbridge_library/src/rosbridge_library/capabilities/advertise_action.py

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