enrich WebVoc context; host context and ontology

As raised in https://github.com/gs1/EPCIS/issues/244:

GS1 voc returns neither the context nor the ontology as pure JSONLD. Instead, both the ontology URL (https://gs1.org/voc) and any term URL (eg https://gs1.org/voc/AdditionalProductClassificationDetails), with any content type, return a HTML, with the whole ontology as embedded JSONLD script. I don't see how to get the context ONLY.

curl -LI -Happlication/ld+json https://www.gs1.org/voc/
HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8

https://gs1.org/voc must support context negotiation and return the ontology as pure JSONLD if requested, and other common serializations (Turtle is most useful, and RDF/XML is mandated)

The WebVoc context is currently very simple, it's just a list of prefixes.

the ontology defines properties, but the context doesn't define any datatypes.

"@graph": [
    {
        "@id": "gs1:referencedFileEffectiveEndDateTime",
        "@type": [
            "rdf:Property",
            "owl:DatatypeProperty"
        ],
        "rdfs:range": {
            "@id": "xsd:dateTime"
        }
  }
]

this means that Voc JSONLD data should include explicit datatypes in order to satisfy the range, eg:

"gs1:referencedFileEffectiveEndDateTime": {"@value": "2021-04-23T17:39:00", "@type": "xsd:dateTime"}

maybe it's better to follow the schema.org approach and define

"@context": {
"gs1:referencedFileEffectiveEndDateTime": {"@type": "xsd:dateTime"}
}

then the user can write simply:

"gs1:referencedFileEffectiveEndDateTime": "2021-04-23T17:39:00"

Currently there's not much benefit in exposing a Voc context. But if it's made richer then:

There should be a defined Voc context URL (following feedback in https://github.com/gs1/EPCIS/issues/244, that'll probably be https://ns.gs1.org/ref/voc-context with optional extension .jsonld)
that should return the context only
- returning ontology triples in the same JSONLD is allowed, but will slow down processing and may compromise caching, so is not recommended
- since it uses schema terms, it should probably include "http://schema.org" (or "https://schema.org") as first element. This shows that URL returns the schema context.
```
curl -LI -Happlication/ld+json http://schema.org
```

gs1 / WebVoc

enrich WebVoc context; host context and ontology #21