Expose the QueryJob on the DBAPI cursor

[r1b]

Is your feature request related to a problem? Please describe.
I would like to store the job_id that references the job created after cursor.execute is called. Currently I do this:

job_id = dbapi_cursor._query_job.job_id

Describe the solution you'd like
I would prefer to do something like this:

# Returns the `QueryJob` created by the last `execute*()` call
job = dbapi_cursor.job()
job_id = job.job_id

Describe alternatives you've considered
It is possible to specify the job_id when calling cursor.execute, however:

• I don't call cursor.execute in my code (see additional context below)
• I would prefer to use the generated ID

Additional context
I interact with this library from the BigQuery SQLAlchemy dialect, so I never call DBAPI execute directly:

sqlalchemy_cursor = engine.execute(...)  # eventually calls `dbapi_cursor.execute`
dbapi_cursor = sqlalchemy_cursor.cursor
job_id = dbapi_cursor._query_job.job_id