Add contexts flattening

[chuwy]

If user defines a context with array root type, then in the resulting LoaderRow we get following structure:

"contexts_com_acme_mycontext_1_0_0": [
  [
    {
      "key": "foo",
      "value": "bar"
    },
    {
      "key": "one",
      "value": "two"
    }
  ]
]

First-level array because contexts always have zero-or-more cardinality and second-level array is actual context.

This structure is not supported by BigQuery, which means we have to flatten this array into single-level.

[chuwy]

If we add this flattening - the table's schema won't be changed (if somebody already pushed such schema), so it can wait.