add API to trigger MTU negotiation

features/FEATURE_BLE/ble/GattClient.h

@@ -635,6 +635,27 @@ public:
         (void) characteristic;
     }
 
+    /**
+     * Trigger MTU negotiation. This might result in a Gap event onAttMtuChange
+     * being called if MTU changes.
+     *
+     * @note This does not guarantee a change in MTU size. If size remains
+     * unchanged no event will be generated.
+     *
+     * @param connection Connection on which the MTU is to be negotiated.
+     *
+     * @return BLE_ERROR_NONE if the procedure has been launched successfully
+     * otherwise an appropriate error.
+     */
+    virtual ble_error_t negotiateAttMtu(
+        Gap::Handle_t connection
+    ) {
+        /* Requesting action from porter(s): override this API if this
+           capability is supported. */
+        (void) connection;
+        return BLE_ERROR_NOT_IMPLEMENTED;
+    }
+
     /**
      * Register an handler for Handle Value Notification/Indication events.
      *

features/FEATURE_BLE/ble/generic/GenericGattClient.h

@@ -111,6 +111,13 @@ public:
         const DiscoveredCharacteristic& characteristic
     );
 
+    /**
+     * @see GattClient::negotiateAttMtu
+     */
+    virtual ble_error_t negotiateAttMtu(
+        connection_handle_t connection
+    );
+
 	/**
 	 * @see GattClient::reset
 	 */

features/FEATURE_BLE/source/generic/GenericGattClient.cpp

@@ -1259,6 +1259,12 @@ void GenericGattClient::terminateCharacteristicDescriptorDiscovery(
 
 }
 
+ble_error_t GenericGattClient::negotiateAttMtu(
+    connection_handle_t connection
+) {
+    return _pal_client->exchange_mtu(connection);
+}
+
 ble_error_t GenericGattClient::reset(void) {
 
 	// _is_reseting prevent executions of new procedure while the instance resets.