Improve typing for plugins

[awaelchli]

What does this PR do?

Follow up to #10163
Requested by #10682
Part of #7037

cc @Borda @justusschock @kaushikb11 @awaelchli @akihironitta @ananthsub @daniellepintz

What does this PR do?

Fixes

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

[tchaton]

LGTM !