Create a link for the best checkpoint

Nobody has claimed this yet.

### Description & Motivation

As illustrated in #19294 , since we already implement the link of `last.ckpt`, how about adding a new feature on `best.ckpt`? Although we support saving the topk checkpoints already, there is no consistent way to reference the top1 checkpoint.

For example, in the above figure, I have to check the files under the folder `work_dirs/medclip_resnet50_iu_xray_R2GenGPT_30x/2024.01.16_17.44.40.927262/checkpoints` to determine the file name `epoch:5-val_loss:3.264.ckpt` to reference the best checkpoint if I have not created the `best.ckpt` checkpoint. It will be more convenient if we implement the hard link of `best.ckpt` also. It can be implemented with the similar code as the `last.ckpt` part.

### Pitch

Create a link for the best checkpoint when it is updated.

### Alternatives

_No response_

### Additional context

_No response_

cc @borda @carmocca @awaelchli

Start by locating the existing `last.ckpt` link implementation and the code that updates top-k checkpoints. Trace how the top1/best checkpoint is selected, then verify that a `best.ckpt` hard link is created or updated whenever that checkpoint changes.

Written by the indexing model from the issue text.