BigQuery REST v2 API

[dbolduc]

We need to produce an idiomatic C++ API surface for https://cloud.google.com/bigquery/docs/reference/rest

This is a big undertaking. It will have several subtasks, such as #13892. We will fill in the details later.

• Disambiguate rest_internal::Post calls when the Response and Request types are the same. #14382
• Generate libraries for new BigQuery services. #14452
• Add integration tests #14456