Add ```annotation_tool``` for ```as-cell-summaries```

Add buckets for 3 annotation tools to as-cell-summaries, similar to what we already have for dataset-cell-summaries. This is to enable more detailed HRApop usage and, later on, comparison of different cell type populations at the anatomical structure level based on annotation tools. These summaries may be merged in the future if a sufficient threshold of agreement is detected (same for dataset-cell-summaries.

x-atlas-consortia / hra-pop

Add ```annotation_tool``` for ```as-cell-summaries``` #6