Expose PlotGeometry in public API (#4193)

Expose `egui_plot::items::values::PlotGeometry` in public API so that
`PlotItem`, which is already public, can actually be implemented by
applications. Fixes #3464.

---------

Co-authored-by: Dominique Würtz <dom@blaukraut.info>

crates/egui_plot/src/plot_ui.rs

@@ -116,6 +116,11 @@ impl PlotUi {
         self.last_plot_transform.value_from_position(position)
     }
 
+    /// Add an arbitrary item.
+    pub fn add(&mut self, item: impl PlotItem + 'static) {
+        self.items.push(Box::new(item));
+    }
+
     /// Add a data line.
     pub fn line(&mut self, mut line: Line) {
         if line.series.is_empty() {