Support templating name of files stored via Rclone backend

[dalbani]

What problem are we solving?

Although the Rclone backend implemented in #4402 works fine, the current implementation has the limitation of storing all files at the root of a Rclone remote, next to each other in the same "folder".
In a SeaweedFS deployment like mine with, hmm, tens of thousands of volume files, I'd like to have a way to split all those files into subfolders.

How are we solving the problem?

By introducing a new configuration property that lets the user provide a template generating the filename from the volume UUID.

How is the PR tested?

Manually, by configuring such a backend:

[storage.backend]
[storage.backend.rclone.default]
enabled = true
remote_name = "onedrive"
key_template = "SeaweedFS/{{ slice . 0 2 }}/{{ slice . 2 4 }}/{{ slice . 4 }}"

And checking that volume files were uploaded in OneDrvive in such a structure:

SeaweedFS/
  5f/
    e5/
      03d8-0aba-4e42-b1c8-6116bf862f71

Which matches the output of volume.list:

...
volume id:8  
  size:106774344
  collection:"bucket-1"
  ...
  read_only:true
  version:3
  modified_at_second:1687612319
  remote_storage_name:"rclone.default"
  remote_storage_key:"SeaweedFS/5f/e5/03d8-0aba-4e42-b1c8-6116bf862f71" 
...

Checks

• I have added unit tests if possible.
• I will add related wiki document changes and link to this PR after merging.

Other considerations

If this template functionality is deemed useful for the Rclone backend, I propose to implement it in a generic fashion, so that the S3 backend can benefit from it as well.

[dalbani]

Thanks for this quick reaction.

FYI, I have just documented this new functionality in https://github.com/seaweedfs/seaweedfs/wiki/Cloud-Tier#rclone.