Add first AreaMetadata with config option and tests.

[lehmann-4178656ch]

This PR adds a switch to add the number of outer polygons for areas.

This information was partially requested by @jogoe222 and allows SPARQL queries selecting complex areas without parsing the WKT representation.

[patrickbr]

Thanks, Axel! I am a bit confused by the relation name multipolygon_outer_polygon_count though - why is "outer polygon" mentioned there explicitly? This seems redundant, as a "normal" polygon can only have a single outer polygon. I think something simpler like outer_polygon_count or even polygon_count or num_polygons would be better here :)

Also, is this really limited to multipolygons like the name suggests? Shouldn't it also be written for "normal" polygons to ensure that a user can filter simple polygons without parsing the WKT?

[lehmann-4178656ch]

I'm not good at naming things, as we only have a global meta prefix I thought that mentioning multipolygons could help the understanding of what this value is about without looking at the subject. But this can and should be improved, currently I like outer_polygon_count as we could add inner_polygon_count in the future. I'll think about this and ask which additional values could be useful -> I'll mark this PR as draft for the meantime.