vfs: option to cache metadata only

What is your current rclone version (output from rclone version)?

1.54

What problem are you are trying to solve?

Requested by @Dibbyo456 in https://github.com/rclone/rclone/issues/4097#issuecomment-798190622

Can we cache metadata only in VFS?

How do you think rclone should be changed to solve that?

Proposed by @ncw in https://forum.rclone.org/t/how-to-cache-only-metadata-on-cache-remote/14846/7

• Enable async writeback in a secure way
• Enable --vfs-cache-mode full not to download the entire file immediately
• Persistent metadata caching (needs a bit more work but isn't nearly as tricky!)

@ivandeex
As an option, consider new --vfs-cache-mode metadata ??

Informative references

• https://forum.rclone.org/t/how-to-cache-only-metadata-on-cache-remote/14846/5
• https://rclone.org/commands/rclone_mount/#vfs-file-caching
• https://github.com/rclone/rclone/blob/v1.54.1/vfs/file.go