Request: Make `_VERSIONED_PATH` configurable via public API

[KahnSvaer]

Description:

Currently, Objaverse uses a private variable _VERSIONED_PATH to determine where data is stored.
To override this behavior, users need to monkeypatch:

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:

oxl.set_data_dir("./data")

or alternatively allow passing a download_dir parameter in relevant APIs.

Optionally, support an environment variable like:

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.