Extend `--static-challenge` to support concat mode like OpenVPN

OpenVPN 2.x recently added support for extending the --static-challenge option with a third argument (scrv1 or concat) to specify whether the password and response should be concatenated or use the SCRV1 protocol (default: scrv1).

This allows simpler handling of password+response in some server-side scripts/plugins that expect plain concatenation instead of the SCRV1-formatted string.

OpenVPN3 currently supports --static-challenge with only two arguments (challenge text and echo flag), but lacks this third argument for format selection.

https://github.com/OpenVPN/openvpn/commit/6f6a0f362f845f042a965f797b731f8931310372
https://gerrit.openvpn.net/c/openvpn/+/665
https://patchwork.openvpn.net/project/openvpn2/patch/20240719131407.75746-1-frank@lichtenheld.com/#6530
https://github.com/OpenVPN/openvpn-gui/issues/663

Example Usage

--static-challenge "Enter OTP:" 0 concat

This would prompt for the challenge response without echo and concatenate it directly with the password.

Why Needed

• Compatibility with servers expecting concatenation behavior.
• Parity with upstream OpenVPN features for easier migration.

Start by locating the OpenVPN3 entry point that parses and handles --static-challenge, then compare its current two-argument behavior with the linked OpenVPN 2.x changes. Support the documented scrv1 default and concat form while preserving existing usage; done means the three-argument example works with the expected password and response format.