Annotations: Legend rendering

**Why is this needed**: Annotation data does not currently render in visualization legends, which means valuable information is not being displayed. We should display data about annotations, organized by color, in the legend. This will not support toggling out of the box as that will require some technical modifications. This feature is the result of a hackathon. Please see https://github.com/grafana/grafana/pull/111880 for preliminary work on this. See also the parent PR that contains other hackathon work https://github.com/grafana/grafana/pull/111691