jsrinnn

BigQuery V2 Job API: Move the MockBigQueryJobConnection to non internal namespace

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Move MockBigQueryJobConnection from `bigquery_v2_minimal_internal` to `bigquery_v2_minimal_mocks` namespace when the api is ready to go public. Please see this code review feedback for reference https://reviewable.io/reviews/googleapis/google-cloud-cpp/11037#-NQVNRXKlkAO-JKQLud--r1-23

This cleanup is result of code review request for PR-14384 . More details [here](https://github.com/googleapis/google-cloud-cpp/pull/14384#pullrequestreview-2146018253). Basically, an `is_null` parameter was added to[ `ColumnData` struct](https://github.com/googleapis/google-cloud-cpp/blob/8d01ae34c018c34a1002187705385775e70a4113/google/cloud/bigquery/v2/minimal/internal/common_v2_resources.h#L317) to distinguish between empty strings and null...