Fix MaxID logic for GCSToBigQueryOperator

[patricker]

related: #26283
closes: #26767

The max_id_key parameter, when used, causes an XCom serialization failure when trying to retrieve the value back out of XCom. This is because instead of storing a single column value in XCom, we were accidentally storing the entire Row.

The Unit Test was updated to reflect the return type of get_job().result(). This operation actually returns a Row iterator, but returning an array of Row works well for the test.

[uranusjr]

With the caveat that I have absolutely no idea whether the old or new logic is correct, lacking knowledge on the API.

[patricker]

@uranusjr If you have access to GCP/BigQuery, this was the quick/simple script I used to test out all the logic first. https://cloud.google.com/bigquery/docs/samples/bigquery-query-results-dataframe (just removed the dataframe part, and played around with the iterator/row access)

[potiuk]

Cool. You can also apply the suggestion from @bhirsz .
I am going to merge that one after we release the other providers (and prepare rc2 of the google provider).