REST search method: ask for preflabels in all languages

[osma]

The REST search method currently returns the prefLabel of the concept in at most two languages (lang and labellang).

Web developers at the National Library of Finland who are using the method for e.g. autocomplete functionality in a web CMS have requested to get all the languages in a single call.

This could be implemented by adding another extension using the fields parameter: fields=prefLabels would cause a new property to be added to the result objects, e.g.

{
    "uri": "http://www.yso.fi/onto/yso/p19378",
    "type": [
        "skos:Concept"
    ],
    "localname": "p19378",
    "prefLabel": "cat",
    "lang": "en",
    "vocab": "yso",
    "prefLabels": {
        "fi": "kissa",
        "sv": "katt",
        "en": "cat"
    }
},