Add TLS-PSK authentication support via callback mechanism

[etr]

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Issue or RFC Endorsed by Maintainers

Supersedes #333 - reimplementation with merge conflicts resolved.

Description of the Change

This PR adds TLS-PSK (Pre-Shared Key) authentication support to libhttpserver via a callback mechanism. PSK allows secure TLS connections without certificates by using a shared secret key.

Key changes:

• Add psk_cred_handler_callback typedef: std::function<std::string(const std::string&)>
• Add psk_cred_handler() builder method to create_webserver
• Implement psk_cred_handler_func() static callback that integrates with GnuTLS
• Add MHD_OPTION_GNUTLS_PSK_CRED_HANDLER option when PSK is configured
• Add AM_CONDITIONAL([HAVE_GNUTLS],...) for conditional compilation in Makefiles
• Remove deprecated AC_HEADER_STDC macro from configure.ac

Usage:

std::string psk_handler(const std::string& username) {
    if (username == "client1") return "0123456789abcdef";  // hex-encoded PSK
    return "";  // empty = reject
}

webserver ws = create_webserver(8080)
    .use_ssl()
    .cred_type(http::http_utils::PSK)
    .psk_cred_handler(psk_handler)
    .https_priorities("NORMAL:+PSK:+DHE-PSK");

Alternate Designs

The callback could return binary data instead of hex-encoded strings, but hex encoding was chosen for simplicity and to match the common PSK representation format used by tools like gnutls-cli.

Possible Drawbacks

• Requires GnuTLS to be available at compile time (guarded by HAVE_GNUTLS)
• PSK is less common than certificate-based TLS, so this adds complexity for a niche use case

Verification Process

• Built with GnuTLS enabled: ./bootstrap && mkdir build && cd build && ../configure && make
• All 12 existing tests pass: make check
• New code passes cpplint style checks
• Added minimal_https_psk example demonstrating the feature

Release Notes

Added TLS-PSK (Pre-Shared Key) authentication support via the new psk_cred_handler() builder method, allowing secure connections without certificates.