[UX] Support launching multiple Kubernetes clusters with sky local up

[DanielZhangQD]

Fix #4794
Support launching multiple Kubernetes clusters with sky local up --k8s-clusters clusters.yaml and cleanning up with sky local up --k8s-clusters clusters.yaml --cleanup.
Example yamls:

ssh_user: admin
ssh_key_path: key.pem
clusters:
  cluster1:
    ips:
    - 5.211.229.79
    - 4.202.228.204
  cluster2:
    ips:
    - 5.207.254.152
    - 5.91.94.121

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
  • Launch local kind cluster
  • Launch 1 remote cluster without password with --ips
  • Launch 1 remote cluster with password with --ips
  • Launch 2 remote clusters without password with --k8s-clusters
  • Launch 2 remote clusters with password with --k8s-clusters
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)

[DanielZhangQD]

I will update the doc after the new option and YAML schema are finalized.