[k8s] Add skypilot-user label to pods

[romilbhardwaj]

Adds skypilot-user label to pods to help with observability.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • Manually with sky launch and kubectl describe pod
  • Adhoc smoke tests pytest tests/test_smoke.py::test_autodown --kubernetes
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[Michaelvll]

LGTM! Thanks!