[Core] More robust authentication on remote cluster

[Michaelvll]

SkyPilot relies on whatever default cloud credential is set up on the remote cluster to perform bucket access or autostop after uploading the local credentials or assigning the IAM role for instance. If a user manually setup some credentials on the remote cluster, it can break the flow of the on remote-instance operations of SkyPilot.

We should make this more robust, e.g. make the credential used by Skypilot more declarative.

Reference: #4395 (comment)

Version & Commit info:

• sky -v: PLEASE_FILL_IN
• sky -c: PLEASE_FILL_IN