Add polygon extractor and reducer

[lcjohnso]

The polygon drawing tool is not currently supported via the shape extractor or reducer. It has been a goal since #4 and #5 to support all drawing tools, including the polygon tool.

PR #105 made significant strides, but questions regarding potential annotation format differences between PFE and FEM put this work on pause. It appears that polygon annotations for FEM has settled on the same format used in PFE -- see Polygon/README.md and the two main FEM PRs: zooniverse/front-end-monorepo#2883 and zooniverse/front-end-monorepo#3389. Therefore, can #105 be revived and completed?