Hide NPCs based on requirements

[Yovez]

Is your feature request related to a problem?
A clear and concise description of what the problem is. E.g. "I'm always frustrated when [...]"
I want a way for NPCs to be hidden from a player unless they've completed a quest, at a certain stage of the quest, or other requirements.

Describe a possible solution
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Hide NPCs based on quest completed or permission, or simply put, add a feature that makes it so the NPC is hidden by default unless a player has a permission or a quest done. Make it a toggleable option though of course.

Describe alternatives you've considered (optional)
A clear and concise description of any alternative solutions or features you've considered.
I've implemented a way to do this through EntityHider: https://gist.github.com/dmulloy2/5526f5bf906c064c255e.
I did it in my own plugin but haven't found a way through my own code to make NPCs default to hidden if applicable.

Additional context (optional)
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[SkytAsul]

Unfortunately this is highly dependent of the NPC plugin you are using, since some of them are making completely fake NPCs using packets (znpcs for instance), some other are using real but modified entities (e.g. Citizens)...
I can't think of a way to do that properly.