BigQuery: cast tuple definition into array of strings

[chuwy]

With the tuple definition:

{
  "type": "array",
  "items": [
    {"type": "string"},
    {"type": "integer"}
  ]
}

We cannot know upfront what elements the array will contain, they can be mixed. Right now we cast the whole array into a string, but we can safely assume that even with undefined schema for 2+ elements - we can safely cast it to Array[String] rather than simple String.