generate proto from bigquery schema

[derekperkins]

We're wanting to migrate all our schema interactions to happen proto first, but that's tough because we have lots of BigQuery schemas with no matching proto. It would help us if we could provide the schema json, and use that to create the proto scaffold. In most cases, I think that's a one-time translation, but I think there are also use cases if you don't control a schema, where it would be convenient to keep the proto in sync as the upstream BigQuery schema changes.