BigQuery: QueryJobConfig and other job configuration classes should accept strings for dataset and table references

[tswast]

Just as in the client class, properties such as QueryJobConfig.default_dataset should accept a fully-qualified string for a dataset or table reference.

[tswast]

This is a follow-up to #6055 and #7477, with the goal being to remove the need to ever manually create a DatasetReference or TableReference.