Need to implement some incremental changes to enable scripting support in the BigQuery library.

• Jobs (e.g. scripts) can create child jobs. There's new stats on the Job Statistics like numChildJobs (for the parent), and parentJobId (for the children). There's also a new enum for the statementType (SCRIPT).

• bigquery.jobs.list now supports passing an option to filter by parentJobId.

• JobStatistics now may include ScriptStatistics message, which itself may also contain ScriptStackFrame information.

This has already been implemented in other languages, so feel free to use them as a reference:
In python, googleapis/google-cloud-python#9190 tracks the work to implement. In go, see https://code-review.googlesource.com/c/gocloud/+/44311