[BE] Tasks by STAGE chart to Visualize Flow and Type of Work

Summary

This is for the backend work required for the "Tasks by Stage" chart, which is part of the Visualize Flow and Type of Work feature set and this description is pulled from that spec.

Tasks by type

For the tasks by type chart we allow the users to select the tasks type by selecting which labels they want shown in the chart and also if they want to see issues or MRs.

Changing to 'Merge requests' in the dropdown will also change the description under the title. So from Showing issues and 4 labels it would change to Showing merge requests and 7 labels for example.

This belongs to Epic &1980 (closed).