Add centralized authentication handler (issue #102)

[etr]

Summary

This PR adds a centralized authentication mechanism to libhttpserver that runs before any resource's render method is called. This addresses issue #102 where users had to copy authentication code into every render_* method.

Features:

• auth_handler: A callback that receives the http_request and can return nullptr to allow the request or an http_response to reject it
• auth_skip_paths: A vector of paths that bypass authentication (supports exact match and wildcard suffix like /public/*)

Files changed:

• src/httpserver/create_webserver.hpp - Added auth_handler_ptr typedef and builder methods
• src/httpserver/webserver.hpp - Added member variables and helper declaration
• src/webserver.cpp - Implemented auth check in request pipeline
• test/integ/authentication.cpp - Added 10 new test cases
• examples/centralized_authentication.cpp - New example file
• README.md - Documentation for the new feature

Example Usage

webserver ws = create_webserver(8080)
    .auth_handler([](const http_request& req) -> std::shared_ptr<http_response> {
        if (req.get_user() != "admin" || req.get_pass() != "secret") {
            return std::make_shared<basic_auth_fail_response>("Unauthorized", "MyRealm");
        }
        return nullptr;  // Allow request
    })
    .auth_skip_paths({"/health", "/public/*"});

Test Plan

• All existing tests pass (make check)
• New tests added for:
  • Centralized auth rejection (401)
  • Centralized auth success (200)
  • Skip paths (exact match)
  • Skip paths (wildcard)
  • No partial prefix matching
  • Deep nested wildcard paths
  • POST method with auth
  • Wrong credentials
  • 404 for non-existent resources
  • Default behavior (no auth handler)
  • Multiple skip paths

Closes #102