Access to RangeAnnotation coordinates

**Feature Request**
Feature that provides access to the coordinates of the annotations drawn on the chart (RangeAnnotations).
The current implementation of RangeAnnotations simply draws a colored container on the chart. This restricts how various applications would like to display annotations. Getting access to the annotation coordinates allows for custom annotations to be drawn.

**Describe the solution you'd like**
A callback function that retrieves the list of annotation coordinates used to draw the annotation container on the chart.
The following [PR#970](https://github.com/imaNNeoFighT/fl_chart/pull/970) implements the solution.

**Additional context**
Here are some use cases where getting access to the RangeAnnotation coordinates can help in rendering custom annotations.
The following was implemented with the changes made on this [PR#970](https://github.com/imaNNeoFighT/fl_chart/pull/970)

Review PR #970 first, since the issue identifies it as implementing the requested solution. Verify that the callback exposes the RangeAnnotation coordinates needed for custom annotations and that the documented use cases are covered; the issue's work is done when that behavior is available in the chart library.

Written by the indexing model from the issue text.