Enhance code generation to support enum definitions in the "schemas" section of JSON

[tjerwinchen]

This commit introduces the ability to define enums in the "schemas" section of a JSON file. The code now has the capability to properly parse and handle enum definitions.

Example, we define a Role as an enum

"schemas": {
  ……
  "Role": {
      "id": "Role",
      "type": "string",
      "default": "HARM_CATEGORY_UNSPECIFIED",
      "enum": [
        "user",
        "assistant",
        "function"
      ],
      "enumDescriptions": [
        "User",
        "Chat Assistant",
        "Func"
      ]
    },
    ……
}

The generated code:

    public enum Role: String, Codable, CaseIterable {
        case user // User
        case assistant // Chat Assistant
        case function // Func
    }

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