subscriptions UPDATE merge operational data

An external tool that implements configured subscriptions can fill other
leaves by subscribing the operational data with the
SR_SUBSCR_OPER_MERGE flag. For instance, the
configured-subscription-state field defined in the RFC 8639
Link:https://datatracker.ietf.org/doc/html/rfc8639.

src/main.c

@@ -943,7 +943,12 @@ server_data_subscribe(void)
 
     /* operational data */
     SR_OPER_SUBSCR(mod_name, "/ietf-subscribed-notifications:streams", srsn_oper_data_streams_cb);
-    SR_OPER_SUBSCR(mod_name, "/ietf-subscribed-notifications:subscriptions", np2srv_oper_sub_ntf_subscriptions_cb);
+    rc = sr_oper_get_subscribe(np2srv.sr_sess, mod_name, "/ietf-subscribed-notifications:subscriptions",
+            np2srv_oper_sub_ntf_subscriptions_cb, NULL, SR_SUBSCR_OPER_MERGE, &np2srv.sr_data_sub);
+    if (rc != SR_ERR_OK) {
+        ERR("Subscribing for providing \"%s\" state data failed (%s).", mod_name, sr_strerror(rc));
+        goto error;
+    }
 
     /*
      * ietf-netconf-server, ietf-keystore and ietf-trustore handled by ln2