relay: fix timing attack on password authentication (GHSA-vhv8-g2r9-cwcc)

The relay authentication used non-constant-time comparisons (strcasecmp,
strcmp) to verify password hashes and plaintext passwords, allowing an
attacker to derive the expected hash byte-by-byte from response timing
and then authenticate without knowing the password.

- SHA/PBKDF2 hex hash comparisons: normalize the client-supplied hash to
  uppercase and compare in constant time over the fixed expected length.
- Plaintext password comparison: HMAC-SHA256 both passwords with a fresh
  per-call random key and compare the fixed-size MACs in constant time,
  hiding both per-byte timing and the password length.

Add string_memcmp_constant_time helper in core, exposed via the plugin
API. Bump WEECHAT_PLUGIN_API_VERSION accordingly.

src/plugins/plugin.c

@@ -624,6 +624,7 @@ plugin_load (const char *filename, int init_plugin, int argc, char **argv)
         new_plugin->strncasecmp = &string_strncasecmp;
         new_plugin->strncasecmp_range = &string_strncasecmp_range;
         new_plugin->strcmp_ignore_chars = &string_strcmp_ignore_chars;
+        new_plugin->string_memcmp_constant_time = &string_memcmp_constant_time;
         new_plugin->strcasestr = &string_strcasestr;
         new_plugin->strlen_screen = &gui_chat_strlen_screen;
         new_plugin->string_match = &string_match;