[GCP] Add gcp.force_enable_external_ips configuration #15138

Job	Run time
python-test (3.8, tests/unit_tests)	5m 39s
python-test (3.8, tests/test_api.py)	3m 15s
python-test (3.8, tests/test_cli.py)	3m 32s
python-test (3.8, tests/test_config.py)	3m 21s
python-test (3.8, tests/test_global_user_state.py)	3m 21s
python-test (3.8, tests/test_jobs.py)	3m 40s
python-test (3.8, tests/test_list_accelerators.py)	3m 47s
python-test (3.8, tests/test_optimizer_dryruns.py)	5m 9s
python-test (3.8, tests/test_optimizer_random_dag.py)	3m 55s
python-test (3.8, tests/test_storage.py)	3m 26s
python-test (3.8, tests/test_wheels.py)	3m 14s
python-test (3.8, tests/test_jobs_and_serve.py)	3m 27s
python-test (3.8, tests/test_yaml_parser.py)	3m 42s
	49m 28s

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