Fast access to geometry type of a collection

[rouault]

There are some clients which could be made much faster if the return of the /collections/{collectionname} endpoint included the type of geometry (Point/MultiPoint, LineString/MultiLineString, Polygon/MultiPolygon), probably as an optional element, when a collection includes only one such single type (which is a common situation).

[cportele]

Similar situation as in #261. Implementations are free to add this information today. For cases where the features in a collection have a known, homogeneous geometry type it is probably a good idea to make this explicit - even if there is not pre-defined element specified in the core conformance class.

At this stage in the process, this would have to be added either in an extension or in a future revision of the core.

[cholmes]

Extension seems to be a good place to start, and then could migrate to the core once there is good acceptance of it.