OpenSSL ENGINE selection omitted from mTLS connection matching

When an application sets `CURLOPT_SSLENGINE` to select an OpenSSL ENGINE for resolving `ENG`-type client certificate and private key identifiers, the engine selection is not included in the TLS configuration compared during connection reuse. A second easy handle with a different engine but the same …

/* Handle A: uses engine_alpha to resolve "slot0-key" */
curl_easy_setopt(A, CURLOPT_SSLENGINE, "engine_alpha");
curl_easy_setopt(A, CURLOPT_SSLKEYTYPE, "ENG");
curl_easy_setopt(A, CURLOPT_SSLCERTTYPE, "ENG");
curl_easy_setopt(A, CURLOPT_SSLKEY, "slot0-key");
curl_easy_setopt(A, CURLOPT_SSLCERT, …