ENH: Add user-agent string when constructing BigQuery and BigQuery Storage API clients.

[tswast]

Since this was a relatively new addition to the BigQuery client library,
only populate the user-agent for BigQuery when recent-enough versions of
google-cloud-bigquery and google-api-core are installed.

Closes #281.

[tswast]

"nightly" failure is concerning.

__________________ TestReadGBQIntegration.test_zero_rows[env] __________________

self = <tests.system.test_gbq.TestReadGBQIntegration object at 0x7f7a2d683d68>
project_id = 'pandas-gbq-tests'

    def test_zero_rows(self, project_id):
        # Bug fix for https://github.com/pandas-dev/pandas/issues/10273
        df = gbq.read_gbq(
            "SELECT title, id, is_bot, "
            "SEC_TO_TIMESTAMP(timestamp) ts "
            "FROM [publicdata:samples.wikipedia] "
            "WHERE timestamp=-9999999",
            project_id=project_id,
            credentials=self.credentials,
            dialect="legacy",
        )
        empty_columns = {
            "title": pandas.Series([], dtype=object),
            "id": pandas.Series([], dtype=np.dtype(int)),
            "is_bot": pandas.Series([], dtype=np.dtype(bool)),
            "ts": pandas.Series([], dtype="datetime64[ns]"),
        }
        expected_result = DataFrame(
            empty_columns, columns=["title", "id", "is_bot", "ts"]
        )
        expected_result["ts"] = expected_result["ts"].dt.tz_localize("UTC")
>       tm.assert_frame_equal(df, expected_result, check_index_type=False)
E       AssertionError: Attributes are different
E       
E       Attribute "dtype" are different
E       [left]:  float64
E       [right]: object

Looks like we need to add a "STRING": "object" row to the map in _bqschema_to_nullsafe_dtypes