refactor: move _unwrap_model to utils.py

[shamykyzer]

@jim-smith suggested on #431 (thread) that _unwrap_model in instance_based_attack.py would be useful to other attacks and should move to sacroml/attacks/utils.py

Tracking here, will open a separate PR to move it and update the import

[shamykyzer]

The unwrap_model move ended up in PR #431 instead of on its own branch, I'll revert it from there and bring it back through a dedicated PR linked to this issue

[jim-smith]

closed by #459