[Backport/Galactic] Simple IT plugins shutdown

[gbiggs]

• implemented shutdown() for simple publisher and subscriber plugin

• added tests for simple publisher/subscriber shutdown

• Removed unneeded lines in publisher shutdown test

Co-authored-by: Matej Vargovcik vargovcik@robotechvision.com

[gbiggs]

CI:

• Linux
• Linux-aarch64
• Windows

[gbiggs]

The failed check passed in the manual CI run, which suggests a flaky test rather than a broken test. Merging this.