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Future PRs will catch cases where we should run more stages, such as if the cluster is outdated, and add documentation.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

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