Require 'static + Send + Sync for proxy callback info?

https://github.com/mullvad/system-configuration-rs/blob/fd43df6f57de954878d824c14c08b57045e46e76/system-configuration/src/dynamic_store.rs#L35-L44

	/// Struct describing the callback happening when a watched value in the dynamic store is changed.
	pub struct SCDynamicStoreCallBackContext<T> {
	/// The callback function that will be called when a watched value in the dynamic store is
	/// changed.
	pub callout: SCDynamicStoreCallBackT<T>,

	/// The argument passed to each `callout` call. Can be used to keep state between
	/// callbacks.
	pub info: T,
	}

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