Cordio BLE: Add hook to HCI driver and transport.

A friend class living in the namespace ble::vendor::cordio and named CordioHCITransportDriver can be added in applications requiring access to internal data of the HCI driver and HCI transport driver.

This is meant to be internal and not easily exploitable by application code.

features/FEATURE_BLE/targets/TARGET_CORDIO/driver/CordioHCIDriver.h

@@ -58,6 +58,10 @@ struct buf_pool_desc_t {
  *   - Access to the write function of the underlying HCITransport driver.
  */
 class CordioHCIDriver {
+
+    // hook for internal tests and passthrough driver
+    friend class CordioHCIHook;
+
 public:
     /**
      * Construct a new instance of an HCI driver.

features/FEATURE_BLE/targets/TARGET_CORDIO/driver/CordioHCITransportDriver.h

@@ -28,6 +28,10 @@ namespace cordio {
  * It allow the stack to write data in the HCI channel.
  */
 class CordioHCITransportDriver {
+
+    // hook for internal tests and passthrough driver
+    friend class CordioHCIHook;
+
 public:
     /**
      * Driver destructor.