Core: Make build_api_url an instance method.

[busunkim96]

See #8737.

Hmm, that @classmethod looks a little bogus to me. All actual uses of it are via instances:

$ git grep "\.build_api_url"
bigquery/tests/unit/test__http.py:        self.assertEqual(conn.build_api_url("/foo"), URI)
bigquery/tests/unit/test__http.py:        uri = conn.build_api_url("/foo", {"bar": "baz"})
bigquery/tests/unit/test__http.py:        expected_uri = conn.build_api_url("/rainbow")
bigquery/tests/unit/test__http.py:        expected_uri = conn.build_api_url("/rainbow")
core/google/cloud/_http.py:        url = self.build_api_url(
core/tests/unit/test__http.py:        self.assertEqual(conn.build_api_url("/foo"), URI)
core/tests/unit/test__http.py:        uri = conn.build_api_url("/foo", {"bar": "baz", "qux": ["quux", "corge"]})
dns/tests/unit/test__http.py:        self.assertEqual(conn.build_api_url("/foo"), URI)
dns/tests/unit/test__http.py:        uri = conn.build_api_url("/foo", {"bar": "baz"})
dns/tests/unit/test__http.py:        expected_uri = conn.build_api_url("/rainbow")
logging/tests/unit/test__http.py:        expected_uri = conn.build_api_url("/rainbow")
resource_manager/tests/unit/test__http.py:        self.assertEqual(conn.build_api_url("/foo"), URI)
resource_manager/tests/unit/test__http.py:        uri = conn.build_api_url("/foo", {"bar": "baz"})
resource_manager/tests/unit/test__http.py:        expected_uri = conn.build_api_url("/rainbow")
runtimeconfig/tests/unit/test__http.py:        expected_uri = conn.build_api_url("/rainbow")
storage/tests/unit/test__http.py:        expected_uri = conn.build_api_url("/rainbow")
storage/tests/unit/test__http.py:        self.assertEqual(conn.build_api_url("/foo"), URI)
storage/tests/unit/test__http.py:        uri = conn.build_api_url("/foo", {"bar": "baz"})
translate/tests/unit/test__http.py:        self.assertEqual(conn.build_api_url("/foo"), URI)
translate/tests/unit/test__http.py:        uri = conn.build_api_url("/foo", query_params=query_params)
translate/tests/unit/test__http.py:        expected_uri = conn.build_api_url("/rainbow")

[tseaver]

@busunkim96 This is a case where we'd actually like to test the change against the affected API libraries (bigquery, dns, logging, resource_manager, runtime_config, storage, translate), but our heuristic for determining whether to run them can't detect that. Can you verify that they still pass with this change?

[busunkim96]

BigQuery unit test failures seem to be unrelated:

=================================== FAILURES ===================================
___________________ test_bq_to_arrow_array_w_special_floats ____________________
module_under_test = <module 'google.cloud.bigquery._pandas_helpers' from '/tmpfs/src/github/google-cloud-python/bigquery/google/cloud/bigquery/_pandas_helpers.py'>
    @pytest.mark.skipif(pandas is None, reason="Requires `pandas`")
    @pytest.mark.skipif(pyarrow is None, reason="Requires `pyarrow`")
    def test_bq_to_arrow_array_w_special_floats(module_under_test):
        bq_field = schema.SchemaField("field_name", "FLOAT64")
        rows = [float("-inf"), float("nan"), float("inf"), None]
        series = pandas.Series(rows, dtype="object")
        arrow_array = module_under_test.bq_to_arrow_array(series, bq_field)
        roundtrip = arrow_array.to_pylist()
        assert len(rows) == len(roundtrip)
        assert roundtrip[0] == float("-inf")
>       assert roundtrip[1] != roundtrip[1]  # NaN doesn't equal itself.
E       assert None != None
tests/unit/test__pandas_helpers.py:491: AssertionError
=============================== warnings summary =============```

[tseaver]

The BigQuery failure is likely due to a new version of a dependency (pyarrow would be my guess).

The Logging failure is a known flakiness in systest teardown, #8676.

[tseaver]

@busunkim96 Do you want to back out d4f53bf from the PR before merging?

[tseaver]

Also, a rebase would likely fix the BigQuery failure, if you'd like to do that first.