Config: Added [allowed_keys] section for client public key authorization on server

- Added CFG_ALLOWED_KEY struct with binary key storage (64 bytes) for fast memcmp comparison
- Implemented parsing of [allowed_keys] section with allow_all flag and per-key entries
- Added key validation in etcp_connections.c server handler (errorcode=8 for rejected keys)
- Default behavior: deny all if [allowed_keys] section is missing or allow_all=0 with no keys
- Added proper memory cleanup and debug logging for allowed_keys configuration
- Updated all test configs to include [allowed_keys] allow_all=1 for backward compatibility

tests/test_etcp_api.c

@@ -55,7 +55,10 @@ static const char* server_config_content =
     "\n"
     "[server: test]\n"
     "addr=127.0.0.1:9031\n"
-    "type=public\n";
+    "type=public\n"
+    "\n"
+    "[allowed_keys]\n"
+    "allow_all=1\n";
 
 // Client config content (uses ports 9032)
 static const char* client_config_content =