[UX] sky launch --fast

[cg505]

Add a --fast flag to sky launch.
When in "fast" mode: check to see if the cluster name already exists, and if it does, and it's available, only run the stages used in exec (SYNC_WORKDIR and EXEC).

Future PRs will catch cases where we should run more stages, such as if the cluster is outdated, and add documentation.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • Manually tested happy path
  • Manually tested on an autostopped or autodowned cluster.
• New smoke tests test_launch_fast and test_launch_fast_with_autostop.
• All smoke tests: pytest tests/test_smoke.py on AWS and Azure
• Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[romilbhardwaj]

Thanks @cg505!