add labels for RangeAnnotations

I would like to see the possibility to pass a label to a RangeAnnotation like in [charts_flutter](https://google.github.io/charts/flutter/example/line_charts/range_annotation). And maybe an Alignment like start, center and end.

Start by locating the RangeAnnotation implementation and any existing annotation tests, then compare the requested behavior with the linked charts_flutter example. Define completion as supporting an optional label and start, center, or end alignment for RangeAnnotations, with tests covering the new options.

Written by the indexing model from the issue text.