[Server] Add ability to create static attributes.

This allows creating instances of services/characteristics and decriptors without
needing to call the parent createX() methods. This allows creation of the attibutes
within a class or other structure and the attributes can be linked to the parent by
calling the parent addX() method.

Additonally this increases the backward compatibility with the original bluedroid library
to further support user migration.

src/NimBLEDescriptor.h

@@ -43,46 +43,48 @@ class NimBLEDescriptorCallbacks;
  */
 class NimBLEDescriptor {
 public:
-    uint16_t     getHandle();
-    NimBLEUUID   getUUID();
-    std::string  toString();
+    NimBLEDescriptor(const char* uuid, uint16_t properties,
+                     uint16_t max_len,
+                     NimBLECharacteristic* pCharacteristic = nullptr);
 
-    void         setCallbacks(NimBLEDescriptorCallbacks* pCallbacks);
+    NimBLEDescriptor(NimBLEUUID uuid, uint16_t properties,
+                     uint16_t max_len,
+                     NimBLECharacteristic* pCharacteristic = nullptr);
 
-    size_t       getLength();
-    uint8_t*     getValue();
-    std::string  getStringValue();
+    ~NimBLEDescriptor();
+
+    uint16_t              getHandle();
+    NimBLEUUID            getUUID();
+    std::string           toString();
+
+    void                  setCallbacks(NimBLEDescriptorCallbacks* pCallbacks);
+
+    size_t                getLength();
+    uint8_t*              getValue();
+    std::string           getStringValue();
+
+    void                  setValue(const uint8_t* data, size_t size);
+    void                  setValue(const std::string &value);
+    NimBLECharacteristic* getCharacteristic();
 
-    void         setValue(const uint8_t* data, size_t size);
-    void         setValue(const std::string &value);
     /**
      * @brief Convenience template to set the descriptor value to <type\>val.
      * @param [in] s The value to set.
      */
     template<typename T>
-    void         setValue(const T &s) {
+    void setValue(const T &s) {
         setValue((uint8_t*)&s, sizeof(T));
     }
 
 private:
     friend class NimBLECharacteristic;
     friend class NimBLEService;
-    friend class NimBLE2902;
     friend class NimBLE2904;
 
-    NimBLEDescriptor(const char* uuid, uint16_t properties,
-                     uint16_t max_len,
-                     NimBLECharacteristic* pCharacteristic);
-
-    NimBLEDescriptor(NimBLEUUID uuid, uint16_t properties,
-                     uint16_t max_len,
-                     NimBLECharacteristic* pCharacteristic);
-
-    ~NimBLEDescriptor();
-
     static int handleGapEvent(uint16_t conn_handle, uint16_t attr_handle,
                               struct ble_gatt_access_ctxt *ctxt, void *arg);
     void       setHandle(uint16_t handle);
+    void       setCharacteristic(NimBLECharacteristic* pChar);
 
     NimBLEUUID                 m_uuid;
     uint16_t                   m_handle;