Add class implementing a Characteristic User Description Descriptor.

This commit introduces a subclass of GattAttribute that implements the Bluetooth SIG defined descriptor, Characteristic User Description Descriptor defined in Bluetooth Core Specification 5.2, Volume 3, Part G, Section 3.3.3.2

It allows the user to provide a string that will be used as the descriptor's value.

Author: AGlass0fMilk

descriptors/CharacteristicUserDescriptionDescriptor.h

@@ -0,0 +1,54 @@
+/*
+ * Mbed-OS Microcontroller Library
+ * Copyright (c) 2020 Embedded Planet
+ * Copyright (c) 2020 ARM Limited
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License
+ */
+
+#ifndef MBED_OS_EXPERIMENTAL_BLE_SERVICES_DESCRIPTORS_CHARACTERISTICUSERDESCRIPTIONDESCRIPTOR_H_
+#define MBED_OS_EXPERIMENTAL_BLE_SERVICES_DESCRIPTORS_CHARACTERISTICUSERDESCRIPTIONDESCRIPTOR_H_
+
+#include "ble/gatt/GattCharacteristic.h"
+#include "ble/gatt/GattAttribute.h"
+
+/**
+ * Class encapsulating a Characteristic User Description Descriptor (CUDD)
+ *
+ * See Bluetooth Core Specification 5.2, Volume 3, Part G, Section 3.3.3.2
+ *
+ * TODO extend this in the future to support client-writable CUDD (set Writable Auxiliary bit)
+ */
+class CharacteristicUserDescriptionDescriptor : public GattAttribute
+{
+public:
+
+    CharacteristicUserDescriptionDescriptor(const char* user_description) :
+        _user_description(user_description),
+        GattAttribute((const UUID&) UUID(BLE_UUID_DESCRIPTOR_CHAR_USER_DESC),
+        (uint8_t*) user_description, strlen(user_description), strlen(user_description), false)
+    { }
+
+    const char* get_user_description() const {
+        return _user_description;
+    }
+
+private:
+
+    const char* _user_description;
+
+
+};
+
+#endif /* MBED_OS_EXPERIMENTAL_BLE_SERVICES_DESCRIPTORS_CHARACTERISTICUSERDESCRIPTIONDESCRIPTOR_H_ */