BigQuery: add retry for query results iteration

[tseaver]

See: googleapis/google-cloud-python#6136, and the TODO left in the code by @jba:

    def result(self, timeout=None):
        ...
        if self.state is None:
            self._begin()
        # TODO: modify PollingFuture so it can pass a retry argument to done().
        return super(_AsyncJob, self).result(timeout=timeout)

[tseaver]

Progress to date:

• BigQuery: 'QueryJob.result' raises 500, even with 'retry' passed when creating the job google-cloud-python#6301 is a user-reported bug for this same issue.
• api_core: retry transient errors in 'PollingFuture.result'. google-cloud-python#6305 broadened the set of exceptions retried by the underlying future.
• BigQuery: add 'retry' argument to '_AsyncJob.result'. google-cloud-python#6302 actually adds a retry argument to _AsyncJob.result(), but can only pass it through to the _begin call unless we decide to extend the future's interface.