Allow idempotent scheduler plugins to be registered via the RPC

[jacobtomlinson]

The Scheduler.add_plugin has an idempotent flag which can be set to True to ensure a plugin is only registered once the first time. The default behaviour is to overwrite the plugin with new ones from subsequent attempts.

This flag is not exposed to the register_scheduler_plugin RPC method. This PR simply passes that flag through.

[jrbourbeau]

Thanks @jacobtomlinson. Exposing the idempotent keyword seems totally reasonable. Would you mind adding a small test for this change?

Also, this exposes an issue where we'll still call the plugin's .start() method even if idempotent=True. Perhaps we should have some extra logic to return early in that case?

[jacobtomlinson]

Thanks for the review @jrbourbeau.

To address the starting of plugins I added another check an even earlier return. Also added tests for idempotent and non-idempotent plugins.

[jrbourbeau]

Thanks @jacobtomlinson!