feat(schema): Add rdau:P60537 (medium of performance) field

[alastair]

Fixes #42

Useful in MusicComposition and MediaObject to indicate instrumentation

As discussed in this issue, and in slack recently.

Need confirmation that this is correct in the context:
"rdau": "http://rdaregistry.info/Elements/u/"

We decided to use library of congress performance mediums, but this patch doesn't verify that the value of the field is a URL. If we want to add this we should modify additionalTypeFieldTransformer to be a generic is-a-url transformer checker.

Example mutation:

mutation {
  CreateMusicComposition(
        name: "Prelude number 1 in some key"
        contributor: "https://musicbrainz.org"
        creator: "https://github.com/trompamusic/ce-data-import"
        source: "https://musicbrainz.org/work/some-uuid"
        format: "text/html"
    mediumOfPerformance:["http://id.loc.gov/authorities/performanceMediums/mp2013015550"]
) {
identifier
}
}

resulting json-ld output (edited)

{
  "@context": [
    "https://schema.org/",
    {
      "dc": "http://purl.org/dc/terms/",
      "prov": "http://www.w3.org/ns/prov#",
      "skos": "http://www.w3.org/2004/02/skos/core#",
      "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
      "bib": "https://bib.schema.org/",
      "rdau": "http://rdaregistry.info/Elements/u/"
    }
  ],
  "@type": [
    "https://schema.org/MusicComposition"
  ],
  ... lots of items chopped here
  "rdau:P60537": [
    "http://id.loc.gov/authorities/performanceMediums/mp2013015550"
  ],
  "firstPerformance": null,
  "bitrate": null,
  "dc:language": null,
  "dc:source": "https://musicbrainz.org/work/some-uuid",
  ... lots of items chopped here
  "dc:identifier": "5a5f683f-ebe4-4ee0-975e-21170b44f6d6",
  "identifier": "5a5f683f-ebe4-4ee0-975e-21170b44f6d6",
  "image": null,
  "schema_publisher": [],
  "workExampleMediaObject": [],
  "dc:created": "2021-04-05T15:21:55.292000000Z",
  "dc:format": "text/html",
  ... lots of items chopped here
  "name": "Prelude number 1 in some key",
  ... lots of items chopped here
  "dc:contributor": "https://musicbrainz.org",
  "contributor": "https://musicbrainz.org",
  ... lots of items chopped here
}

[musicog]

Almost perfect :-)

The JSON-LD output needs to produce {"@id": "http://example.org/foo"} objects for external URIs, instead of just "http://example.org/foo", which is otherwise interpreted as a string literal.

[alastair]

The correct output of this field should be:

"rdau:P60537": [
    {"@id": "http://id.loc.gov/authorities/performanceMediums/mp2013015550"}
]

We're going to need a way that indicates to the json-ld serialiser that some fields are IDs instead of string constants. This could be added to the json schema definition file. If we do this then it makes sense that we also validate that the field is a URL, because to be an ID it always must be an external url.

[alastair]

I updated this to add a relationalProperties field to the json definition file that indicates when a field should be rendered as a reference to an external url instead of a string.

I also updated the additionalProperties transformer to turn it into a generic "check is a url" transformer and added the mediumOfPerformance field

[alastair]

@ChristiaanScheermeijer ready for review

[ChristiaanScheermeijer]

LGTM