[WIP] Explore stricter projjson schema

[rouault]

• projjson.schema.json: update to draft 2020-12 for JSON Schema, and use 'unevaluatedProperties':false to cleanup inheritance constructs
• projjson.schema.json: introduce sub-classes of coordinate systems, and tighten definition of geodetic_crs and geographic_crs
  by referencing the appropriate subclasses

None of the above changes are supposed to change the fact that existing PROJJSON geodetically sane instances should still validate (at least as shown by my quick testing). They prevent for example an inappropriate subtype of coordinate system to be assigned to a GeodeticCRS/GeographicCRS

Background for that experiment: follow-up to the first meeting (maybe last for me) of the OGC CRS group about a CRS JSON standard: http://lists.ogc.org/pipermail/coordtran.wg/2024-September/002773.html