Before submitting this PR, please make sure:

• Your code builds clean without any errors or warnings
• Tests all pass
• You are using approved title ("feat/", "fix/", "docs/", "refactor/")

A description of the changes proposed in the pull request:
This PR introduces the ability to disable TLS 1.3 in the ConnectionsFactory class to address compatibility issues with VPNs that may block TLS 1.3 connections. Key changes include:

• Refactored to a proper singleton pattern to avoid class attribute shadowing by instance attributes
• Added a set_disable_tls_1_3() method to toggle TLS 1.3 support.
• Modified _get_shared_client() to use TLSv1.2 when TLS 1.3 is disabled.
• Ensured proper cleanup of client sessions when TLS settings change.

Tests performed by the developer:

• Added comprehensive unit tests for the new functionality.
• Verified backward compatibility with existing connections.
• Tested manually with openVPN setup that previously had issues with TLS 1.3.

Tips for QA testing:

• Verify that existing connections (without TLS 1.3 disabled) remain unaffected.
• Check for any performance impact (or just verify it works) when TLS 1.3 is disabled.