Add nginx-style authrequest plugin for HTTP /auth gating

[Copilot]

Implemented nginx auth_request-compatible SSH plugin that calls an HTTP /auth endpoint with Basic Auth derived from SSH credentials; 200 OK continues to the next plugin, any other status fails.

• New plugin: plugin/authrequest with CLI flags (--url, --path, --timeout) and X-Forwarded-For propagation.
• Behavior: Performs GET to ${url}${path} (default /auth), discards response body safely, and on 200 returns NextPlugin auth to continue the chain.
• Docs & tests: Added README describing usage; unit tests validate success and failure flows.

Example:

sshpiperd authrequest --url https://auth.example.com --path /auth

Original prompt

impl auth plugin using same protocol as nginx's external auth
open url and call a /auth to see if 200

auth_request        /auth;

you can take a look at
https://github.com/tg123/sshpiper-gh

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