nimble/host/gap: Add callback for subrate change event

`ble_gap_rx_subrate_change()` handler now
invokes `ble_gap_call_conn_event_cb()` to allow applications to react
to subrate change events.

This addition ensures that registered connection event callbacks are
notified when a subrate change occurs, providing better support
for LE Subrate procedures.

Author: szymon-czapracki

nimble/host/src/ble_gap.c

@@ -1993,6 +1993,7 @@ ble_gap_rx_subrate_change(const struct ble_hci_ev_le_subev_subrate_change *ev)
     event.subrate_change.supervision_tmo = le16toh(ev->supervision_tmo);
 
     ble_gap_event_listener_call(&event);
+    ble_gap_call_conn_event_cb(&event, ev->conn_handle);
 }
 #endif