Request: Make `_VERSIONED_PATH` configurable via public API

### Description:

Currently, Objaverse uses a private variable _VERSIONED_PATH to determine where data is stored.
To override this behavior, users need to monkeypatch:
```python
import objaverse as oxl
oxl._VERSIONED_PATH = "./data"
```
This approach has a few downsides:

- Relies on a private/internal variable
- Not stable across versions
- Not discoverable for users
- Can break unexpectedly if internal implementation changes

### Proposed solution:

Expose a public way to configure the data directory, such as:

```python
oxl.set_data_dir("./data")
```
or alternatively allow passing a download_dir parameter in relevant APIs.

Optionally, support an environment variable like:
```python
OBJAVERSE_DATA_DIR=./data
```
### Benefits:

- Cleaner and safer usage
- Avoids reliance on internal implementation
- Improves usability for large-scale dataset workflows
- Backward compatibility can be maintained by keeping the current default behavior when no directory is specified.