[Programmatic API] Merge `Task.set_storage_mounts` and `Task.set_file_mounts`

[Michaelvll]

Currently, we have both Task.set_storage_mounts and Task.set_file_mounts. It is a burden for the user of programmatic API to manually distinguish the two mounts. Instead, we can align it with our file_mounts in our yaml specs, where both sky storage and normal file_mounts are placed under file_mounts section, i.e. we can only preserve Task.set_file_mounts and Task.update_file_mounts.