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.

• [x] #14382
• [ ] Generate libraries for new BigQuery services.
• [ ] Add biguqery_job to build infrastructure.
• [ ] Add bigquery_admin to build infrastructure.