Adding challenge customization option

[oculos]

The proposed PR adds a challenge option so that a custom challenge prompt can be customized. It replaces the prompt sent on the Reply-Message attribute of an Access-Challenge request.

The main reason for this is that some RADIUS servers, such as Microsoft NPS with Azure MFA extension, does not allow the customization of this message on the server side.

If approved, the next work will be to add the option for adding a suffix to messages sent by the server, as currently the message lacks a ": ", being not very good cosmetically for command line use.

[oculos]

Hi @alandekok - I resubmitted this PR so that all unrelated code was suppressed.