[Spot/Storage] Supports empty spaces in source path for storages

[landscapepainter]

This PR resolves #2763 and #2243.

When a source path specified to file_mounts in task YAML had an empty space, Skypilot failed to upload the source to the cloud storages due to the limitation of CLIs used(#2243). This issue also prevented users from specifying workdir with empty spaces when running managed spot jobs as well since workdir is uploaded to the cloud storage first(#2763).

This PR supports the use of empty spaces in the file/directory names for source in file_mounts and workdir for managed spot jobs. And a test test_upload_source_with_spaces is added to check such cases.

Thanks to @aseriesof-tubes for the contribution.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Manually tested the case mentioned in [Spot] Spot launch with a workdir path that contains spaces fails #2763
• Manually tested the case mentioned in [Storage]Paths with empty space character failing in task yaml #2243
• Relevant individual smoke tests: pytest tests/test_smoke.py::TestStorageWithCredentials
• pytest tests/test_smoke.py::test_gcp_storage_mounts_with_stop --gcp
• pytest tests/test_smoke.py::test_aws_storage_mounts_with_stop --aws
• Manual test with task YAML 1:

file_mounts:
  /doyoung-testing:
    name: doyoung-testing
    source: [~/yaml/doyoung-test.yaml, '/home/doyoung/path with spaces/file with spaces']
    store: gcs

• Manual test with task YAML 2:

file_mounts:
  /gcs-mount: 
    name: source-with-space
    source: /home/doyoung/source with space
    store: gcs

[Michaelvll]

Thanks for fixing the issue @landscapepainter! LGTM. It should be good to go in once the tests for storage have passed.