The `DefaultValue` of a `CustomProperty` is incorrect type

[stevehipwell]

Since GitHub added support for multi-select it's been possible for the DefaultValue of a CustomProperty to be either a string or an array of strings; see schema below.

      "default_value": {
        "oneOf": [
          {
            "type": "string"
          },
          {
            "type": "array",
            "items": {
              "type": "string"
            }
          }
        ],
        "description": "Default value of the property",
        "type": [
          "null",
          "string",
          "array"
        ]
      },

Based on the existing implementation of CustomPropertyValue where Value has the same type we should modify DefaultValue to any.