Add 'QueryJob.query_plan' property.

[tseaver]

Cherry-picked from PR #3721

[tseaver]

@tswast I noted your question about creating a to-be-written QueryPlanEntry class: here, because the values are "dumb data" (the class would have no methods except those to marshall instances from the JSON resource), I think it is better to just leave them as dicts.

This is different for referenced_tables and unused_query_parameters, where we already have domain objects which correspond to those resources.

[tswast]

Note: I still think a class would be useful for string to int conversion, but it was only a P2 requirement from BQ team, so I won't block the PR.

[tseaver]

@tswastdb24010 should address your requested changes.

Note: that commit adds 350 lines of code for very little tangible benefit: I can't imagine anybody writing code which depends on the fact that records_read / records_written are integers.

[tswast]

Thanks!

The new class will definitely be a usability improvement over using dictionaries. I agree that usage will likely be low. That's why it was a P2 item in the requirements doc from the BQ eng team.