check if a Plugin is present

We have `register_plugin` and `unregister_*_plugin`
I would like a function to check if a plugin is registered.

I guess technically this is a bit race conditiony,
but it would be useful.
For example, I have a worker plugin that configures our systems logging.
before running a job i want to check if a client passed to me has had it configured and if not throw an error before i call `client.submit`