[SkyServe] Support mixture of spot and on-demand #13214

Job	Run time
python-test (3.8, tests/unit_tests)	6m 21s
python-test (3.8, tests/test_api.py)	3m 59s
python-test (3.8, tests/test_cli.py)	4m 21s
python-test (3.8, tests/test_config.py)	3m 57s
python-test (3.8, tests/test_global_user_state.py)	3m 59s
python-test (3.8, tests/test_jobs.py)	4m 42s
python-test (3.8, tests/test_list_accelerators.py)	4m 49s
python-test (3.8, tests/test_optimizer_dryruns.py)	6m 2s
python-test (3.8, tests/test_optimizer_random_dag.py)	4m 24s
python-test (3.8, tests/test_storage.py)	3m 54s
python-test (3.8, tests/test_wheels.py)	3m 57s
python-test (3.8, tests/test_spot.py)	4m 47s
python-test (3.8, tests/test_yaml_parser.py)	4m 0s
	59m 12s

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