Add EXAMPLE_OF_WORK relations that link directly to Audio/MediaObject

Because workExample links to CreativeWorkInterface, we can't use the
auto-generated filters to query a more specific type, e.g.:
  MusicComposition(filter:{workExample:{contentUrl_not:null}})
doesn't work because contentUrl is part of MediaObject not CreativeWork.

We add additional fields using the same relation name that point
directly to the concrete types that we use in TROMPA. This means that
the workExample relation still works, but if we need to filter on a
specific Audio/MediaObject field we can. This isn't very sustainable
because we'd have to add it every time we need to do this kind of query
to a specific type, but it's a quick fix for our current requirements

src/schema/type/MusicComposition.graphql

@@ -210,6 +210,10 @@ type MusicComposition implements MetadataInterface & SearchableInterface & Thing
   video: [VideoObject] @relation(name: "VIDEO", direction: "OUT")
   "https://schema.org/workExample"
   workExample: [CreativeWorkInterface] @relation(name: "EXAMPLE_OF_WORK", direction: "IN")
+  "https://schema.org/workExample"
+  workExampleMediaObject: [MediaObject] @relation(name: "EXAMPLE_OF_WORK", direction: "IN")
+  "https://schema.org/workExample"
+  workExampleAudioObject: [AudioObject] @relation(name: "EXAMPLE_OF_WORK", direction: "IN")
   "https://bib.schema.org/workTranslation"
   workTranslation: [CreativeWorkInterface] @relation(name: "WORK_TRANSLATION", direction: "OUT")
   #######################################