Add @context to the eIDAS Link structure

[iamtxena]

When two software systems need to exchange data, they need to use terminology that both systems understand. As an analogy, consider how two people communicate. Both people must use the same language and the words they use must mean the same thing to each other. This may be referred to as "the context of a conversation".

Simply speaking, a context is used to map terms to IRIs. Terms are case sensitive and any valid string that is not a reserved JSON-LD keyword can be used as a term. As referred in json-ld spec

{
  "@context": {
    "name": "http://schema.org/name",
    ↑ This means that 'name' is shorthand for 'http://schema.org/name'
    "image": {
      "@id": "http://schema.org/image",
      ↑ This means that 'image' is shorthand for 'http://schema.org/image'
      "@type": "@id"
      ↑ This means that a string value associated with 'image'
        should be interpreted as an identifier that is an IRI
    },
    "homepage": {
      "@id": "http://schema.org/url",
      ↑ This means that 'homepage' is shorthand for 'http://schema.org/url'
      "@type": "@id"
      ↑ This means that a string value associated with 'homepage'
        should be interpreted as an identifier that is an IRI 
    }
  }
}

Contexts can either be directly embedded into the document (an embedded context) or be referenced using a URL. Assuming the context document in the previous example can be retrieved at https://json-ld.org/contexts/person.jsonld, it can be referenced by adding a single line and allows a JSON-LD document to be expressed much more concisely as shown in the example below:

{
  "@context": "https://json-ld.org/contexts/person.jsonld",
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/",
  "image": "http://manu.sporny.org/images/manu.png"
}

We should then define a @context structure and publish it either embedded or referenced using a URL.