feat(tauri-runtime-wry): add plugin API

[lucasfernog]

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Does this PR introduce a breaking change?

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[JonasKruckenberg]

I think it'd make more sense to make this a callback on the Plugin trait instead of a completely separate thing wdyt?

You could call it on_wry_event or something 🤔

Edit: Or rather, we can have the TauriPlugin struct that is returned by the builder implement tauri_runtime_wry::Plugin too, that way we can just pass the whole plugin through to wry