Store the relative path instead of absolute path in `KvFileHandle`

Currently, we will store the absolute path in `KvFileHandle`, something like:
```
ossj://fluss-xxx/remote-dir/kv/fluss_db/flusstable-0/20250304-p8/105/snap-64
```
Actual, we can just store `snap-64` in `KvFileHandle`, and make `ossj://fluss-xxx/remote-dir/kv/fluss_db/flusstable-0/20250304-p8/105` stored in `CompletedSnapshot`