Make GCS-style checkpointing configurable instead deciding it on is_gcs_path function output

Hi, i'm using s3fs which has very similar restrictions as GCS (no atomic moves, for example).
To enable GCS-style checkpointing I'm patching orbax which seems very ugly

```
def patch_with_gcs_style_checkpointing_for_path_prefix(prefix):
def is_gcs_path(path: str) -> bool:
return os.fspath(path).startswith(prefix)

ocp.utils.is_gcs_path = is_gcs_path

patch_with_gcs_style_checkpointing_for_path_prefix("/mnt/s3")
```

Instead, I would prefer to have a parameter in `ocp.CheckpointManagerOptions` that would enable this checkpointing style.
Its default value may be False and in `ocp.CheckpointManager.__init__` there could be `self._gcs_style_checkpointing = gcs_style_checkpointing or is_gcs_path(directory)` which would be totally backward compatible.