update openapi/jsonschema conversion to translate jsonUnknown to additionalProperties

[benoitlouy]

Given the following smithy definition

structure Dog {
  name: String
  @jsonUnknown
  attributes: Attributes
}

map Attributes {
  key: String
  value: Document
}

the following JSON schema will be produced

"Dog": {
  "type": "object",
  "properties": {
    "name": {
      "type": "string"
    }
  },
  "additionalProperties": true
}