alpn: query filter

Add a connection filter query to obtained the negotiated ALPN
protocol to check in setup/protocols how the connection needs
to behave.

Remove the members `alpn` and `proxy_alpn` from `connectdata`.

Closes #17947

lib/cf-h2-proxy.c

@@ -1534,6 +1534,12 @@ static CURLcode cf_h2_proxy_query(struct Curl_cfilter *cf,
     }
     break;
   }
+  case CF_QUERY_ALPN_NEGOTIATED: {
+    const char **palpn = pres2;
+    DEBUGASSERT(palpn);
+    *palpn = NULL;
+    return CURLE_OK;
+  }
   default:
     break;
   }