Support pinning config repo

It's nice to be able to pin the config repo by commit hash or tags.

This can be achieved by one of these solutions.

```yaml
 - uses: fuxingloh/multi-labeler@v4
   with:
     config-repo: my-org/my-repo@my-ref
```

or

```yaml
 - uses: fuxingloh/multi-labeler@v4
   with:
     config-repo: my-org/my-repo
     config-ref: my-ref
```
