[SkyServe] Support mixture of spot and on-demand & Spot policy #11251

Job	Run time
python-test (3.8, tests/unit_tests)	4m 49s
python-test (3.8, tests/test_api.py)	2m 35s
python-test (3.8, tests/test_cli.py)	2m 29s
python-test (3.8, tests/test_config.py)	2m 20s
python-test (3.8, tests/test_global_user_state.py)	2m 23s
python-test (3.8, tests/test_jobs.py)	2m 55s
python-test (3.8, tests/test_list_accelerators.py)	2m 22s
python-test (3.8, tests/test_optimizer_dryruns.py)	4m 15s
python-test (3.8, tests/test_optimizer_random_dag.py)	3m 34s
python-test (3.8, tests/test_storage.py)	2m 36s
python-test (3.8, tests/test_wheels.py)	2m 21s
python-test (3.8, tests/test_spot.py)	2m 28s
python-test (3.8, tests/test_yaml_parser.py)	2m 26s
	37m 33s

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