Add field to describe API access

[nichtich]

By now we use the non-standard field API. Better add servicealigned with dcat:DataService. Maybe this:

{
  "service": [
  {
    "apitype": "http://bartoc.org/api-type/skosmos",
    "endpoint": "https://lod.gesis.org/iso-3166-1/"
  },
  {
    "apitype": "http://bartoc.org/api-type/sparql",
    "endpoint": "https://publications.europa.eu/webapi/rdf/sparql"
  },
  {
    "apitype": "http://bartoc.org/api-type/skosmos",
    "endpoint": "https://skosmos.bartoc.org/951/"
  }
  ]
}

<> dcat:service [ a dcat:DataService ;
    dcterms:conformsTo <http://bartoc.org/api-type/skosmos> ;
    dcat:endpointURL "https://lod.gesis.org/iso-3166-1/"
 ],[ a dcat:DataService ;
    dcterms:conformsTo <http://bartoc.org/api-type/sparql> ;
    dcat:endpointURL "https://publications.europa.eu/webapi/rdf/sparql"
 ],[ a dcat:DataService ;
  dcterms:conformsTo <http://bartoc.org/api-type/skosmos> ;
  dcat:endpointURL "https://skosmos.bartoc.org/951/"
] .

Open questions:

• how to relate endpoint to specific version/subset/language... of a vocabulary?
• which is the actual endpoint? e.g. for Skosmos it is https://lod.gesis.org/rest/v1/iso-3166-1/ instead of current https://lod.gesis.org/iso-3166-1/. For SPARQL we often refer to a landing page instead of specific endpoint URI (use dcat:landingPage?)