Add a public API for overriding plot legend traces' visibilities

[jayzhudev]

Added a public API in egui_plot -> legend to allow hidden_items to be overridden in the plot legend widget. This allows convenient control of traces' visibilities the selection of traces from the application code.

Example

    let legend_config = if plot_selection_changed {
        let hidden_items = match plot_selection {
            PlotSelection::SelectAll => Vec::new(),
            PlotSelection::DeselectAll => all_trace_names,
        };

        Legend::default()
            .position(Corner::RightTop)
            .hidden_items(hidden_items) // Overrides `hidden_items`
    } else {
        Legend::default().position(Corner::RightTop)
    };

    Plot::new(id)
        .legend(legend_config)
        .show(ui, draw_plot);

Closes #3533.

[jayzhudev]

@emilk Hello, any comment or feedback on this small PR? Thanks 😃