feat: Add spans for plots

[michalsustr]

This PR adds supports for spans in plots on either axis. This allows for example to highlight a region of interest or draw states in the background of plots.

Added items to the public API

+pub struct egui_plot::Span
+impl egui_plot::Span
+pub fn egui_plot::PlotUi<'a>::span(&mut self, span: egui_plot::Span)

[michalsustr]

Contributed by @azerupi in #152