Annotation class should allow extra fields for Web Annotation Data Model

[markpbaggett]

The Web Annotation Data Model describes many properties like created, creator, modified, etc. In order to avoid adding all of these to the presentation-validator's json schema, we should probably just allow extras on instances of the class.

Expected behaviour:

If you create an Annotation like so:

body = ResourceItem1(
  type="TextualBody",
  language="de",
  format="text/plain",
  value="Göttinger Marktplatz mit Gänseliesel Brunnen"
)
annotation = Annotation(
  id="https://iiif.io/api/cookbook/recipe/0266-full-canvas-annotation/canvas-1/annopage-2/anno-1",
  motivation="commenting",
  creator="http://example.org/user1",
  created="2015-01-28T12:00:00Z",
  modified="2015-01-29T09:00:00Z",
  body=body,
  target="https://iiif.io/api/cookbook/recipe/0266-full-canvas-annotation/canvas-1"
)
print(annotation.json(indent=2))

creator, created, and modified should not be dropped.

Observed behaviour:

The print statement drops the properties:

{
  "@context": "http://iiif.io/api/presentation/3/context.json",
  "id": "https://iiif.io/api/cookbook/recipe/0266-full-canvas-annotation/canvas-1/annopage-2/anno-1",
  "type": "Annotation",
  "motivation": "commenting",
  "body": {
    "type": "TextualBody",
    "value": "Göttinger Marktplatz mit Gänseliesel Brunnen",
    "format": "text/plain",
    "language": "de"
  },
  "target": "https://iiif.io/api/cookbook/recipe/0266-full-canvas-annotation/canvas-1"
}

Potential bug location(s):

Easiest fix is to add:

class Config:
        extra = Extra.allow

to Annotation.

Any other comments:

Again, it seems burdensome to try to add all these possible properties to the validation schema, so this seems like a good use of Extra.allow.

[glenrobson]

Maybe add "additionalProperties" to:

https://github.com/IIIF/presentation-validator/blob/b602feb7cbdf8b7922dfa8200dfe21597566898e/schema/iiif_3_0.json#L899