[SkyServe] Support mixture of spot and on-demand & Spot policy #11369

Job	Run time
python-test (3.8, tests/unit_tests)	5m 10s
python-test (3.8, tests/test_api.py)	2m 44s
python-test (3.8, tests/test_cli.py)	3m 2s
python-test (3.8, tests/test_config.py)	2m 29s
python-test (3.8, tests/test_global_user_state.py)	2m 27s
python-test (3.8, tests/test_jobs.py)	2m 59s
python-test (3.8, tests/test_list_accelerators.py)	2m 40s
python-test (3.8, tests/test_optimizer_dryruns.py)	4m 18s
python-test (3.8, tests/test_optimizer_random_dag.py)	3m 31s
python-test (3.8, tests/test_storage.py)	2m 38s
python-test (3.8, tests/test_wheels.py)	2m 40s
python-test (3.8, tests/test_spot.py)	2m 33s
python-test (3.8, tests/test_yaml_parser.py)	2m 31s
	39m 42s

Provide feedback