[Format] Ambiguity with extension and dictionary

[Antoine Pitrou]

Hello,

Let's an IPC Schema message contains the following Field (in pseudo-JSON
representation):

    {
      "name" : "...",
      "nullable" : true,
      "type" : Utf8,
      "dictionary": {
        "id": 0,
        "indexType": Int32,
        "isOrdered": true
      },
      "children" : [],
      "metadata" : [
         {"key": "ARROW:extension:name", "value": "MyExtType"},
         {"key": "ARROW:extension:metadata", "value": "..."}
      ]
    }

Which of the following two logical types does it represent?

- MyExtType<storage = int32-dictionary<string>>
- int32-dictionary<MyExtType<storage = string>

Regards

Antoine.