Add `fn add_boxed(&mut self, Box<dyn PlotItem>)` method to `PlotUi`.

[freeformstu]

Is your feature request related to a problem? Please describe.

We have a somewhat unique setup in which we use dioxus for state management and egui for all of the underlying component rendering. When passing attributes through dioxus we have to cast the incoming type which requires that the trait objects be "object safe". PlotItem is not object safe, so we can't pass it through directly. But we also cannot pass in our own trait which has a method such as fn as_egui_plot_item(&self) -> impl PlotItem because that is also not object safe so instead we have a trait which has a method fn as_egui_plot_item(&self) -> Box<dyn PlotItem>. This cannot be passed to the existing fn add(&self, impl PlotItem) method because Box<dyn PlotItem> does not satisfy the impl PlotItem argument.

Describe the solution you'd like

Add an fn add_boxed(&mut self, Box<dyn PlotItem>) method to allow insertion directly into the underlying plot_items Vec.

Describe alternatives you've considered

1. Make PlotItem object safe (likely a pretty large effort and I'm not sure anyone else cares about this).
2. impl<T: PlotItem> PlotItem for Rc<T> so we can pass an Rc directly to the .add( function. (This could also be done for Arc and others for convenience).
3. Possibly some more options?

Additional context