feat(bigquery): add script statistics to job resource

[tswast]

In addition to the parent ID property, the new scripting feature adds a new scriptStatistics property. This can be calling jobs.get on a script child job in the APIs Explorer.

• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Follow-up to #9190 🦕

[tseaver]

Please add explicit unit test coverage for the ScriptStackFrame and ScriptStatistics classes.

[shollyman]

Iterating by parent was already implemented, as were the ParentJobId and NumChildJobs fields on JobStatistics. This rounds out the implementation. I am assuming integration testing will happen in the context of samples.