Fix Faulthandler import error for RobotFramework (RF).

[Guiliano99]

Add optional faulthandler import via environment flag for debugging

Description

• Adds environment flag PYOQS_ENABLE_FAULTHANDLER to control the optional import of the Python faulthandler module.
• Prevents import errors during Robot Framework (RF) test execution when faulthandler is unavailable or unsupported.
• Retains improved debugging capabilities for segmentation faults when the flag is enabled.

Motivation and Context

While running RF tests with the cmp-test-suite using liboqs-python, an import error occurred for the faulthandler module.
This module was originally added to improve debugging for segmentation faults caused by missing build-time flags ( when PQC stateful hash signature key generation support was not enabled in liboqs).

To maintain this helpful debugging functionality without breaking test environments, an environment variable PYOQS_ENABLE_FAULTHANDLER was introduced.
When set, it enables faulthandler explicitly; otherwise, the module is skipped gracefully.