Add support for ondemand cluster default creds

[carolineechen]

perform some extra setup to allow for using your own default creds for ondemand clusters.

• if ~/.ssh/sky-key exists: do nothing, sky will launch with those creds
• if ~/.ssh/sky-key does not exist and default_ssh_key exists, copy those creds to ~/.ssh/sky-key so sky can launch with those creds
• if neither exists, do nothing. sky will create the creds when it launches

note that this does not support passing in custom ssh_creds for ondemand_clusters. it's only for the case of defaulting the sky-key to your default key if you have a default key but no sky key.

[carolineechen]

• Add support for ondemand cluster default creds #1359 👈
• Use default secret for static clusters #1358
• Refactor writing down cluster creds #1344
• Split cluster keys and ssh properties #1338
• Update cluster saving and deleting secrets #1349
• main

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