2261 on terminate agents signal

[cakekoa]

What does this PR do?

Fixes part of #2261.

Implements on_terminate_agents_signal, and connects the AgentSignalsService to the event.

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
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• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
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  Tested by {Running the Monkey locally with relevant config/running Island/...}

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