Support object enum

[ben-manes]

JsonSchema allows for enumerations of objects, not just strings. For complex workflows this is very beneficial, e.g.

"status": {
  "type": "object",
  "properties": {
    "foo": {
      "type": "string",
    },
    "bar": {
      "type": "string",
    }
  },
  "enum": [
    { "foo": "1", "bar": "A" },
    { "foo": "2", "bar": "B" },
    { "foo": "3", "bar": "C" }
  ]
}

Unfortunately the generated code will look something like,

enum Status {
  __EMPTY__(null),
  __EMPTY___(null),
  __EMPTY____(null);

  ...
}

For now this cannot be done by type-safe generated code, but manually kept in sync.

[tillias]

+1
Can be solved from business perspective as #1193 (comment) with oneOf, but generator will still generate empty class :(

[unkish]

• in case of enum definition without extensions as provided in sample - what would the expected enum constant names be instead of __EMPTY__ ?
• how would serialized form look like ?
  • enum constant name (as with non-object enums)
  • object form (enum properties)
    • would that be a jackson-json specific feature or other JSON libraries like moshi, gson, etc. are able to support enum object serialization/deserialization "out of the box" ?
  • some other form ?
• any restrictions that might exist ?