Calculate and return a polygon based on the vds trace mask

The polygon of the bounding box of the volume is trivially available as metadata in the vds through entries such as origin, rotation, spacing and number of samles.

More useful however, might be the the polygon of the trace mask. I.e. where the volume contains data. This would include cut of edges and corners in irregular sized cubes as well as holes.

This should be and entry in the /metadata endpoint. The representation should match common polygon representation for easy integrations with other formats. E.g. PostGis defines them like this:

```
'Polygon', 'POLYGON((0 0, 1 0, 1 1, 0 1, 0 0))',
'PolygonWithHole', 'POLYGON((0 0, 10 0, 10 10, 0 10, 0 0),(1 1, 1 2, 2 2, 2 1, 1 1))',
```

For us a list of points might make more sense than a string, but we can reuse the shape.