Register of x-ogc-* extensions to JSON Schema vocabulary

[jerstlouis]

OGC API - Features Part 5: Schemas, which allows providing a logical schema for a collection of data, defines some extensions to the JSON Schema vocabulary in order to qualify the properties / fields of the data with annotation such as for example a semantifc definition.

We also hope for the Common aspects of Schems to become a Part 3? of OGC API - Common, since it applies not only to Features, but potentially to all OGC API data access.

I set up this Wiki page here to keep track of these extensions until we have a Naming Authority register and/or to keep track of new recently proposed annotations.

See also opengeospatial/ogcapi-features#945 .