[BREAKING] Refactor attributes

Refactor attributes to reduce code duplication and improve maintainability.

* Add attribute base classes to provide common code.
* Add const where possible to functions and parameters.
* `NimBLECharacteristic::notify` no longer takes a `bool is_notification` parameter, instead `indicate()` should be called to send indications.
* `NimBLECharacteristic::indicate` now takes the same parameters as `notify`.
* `NimBLECharacteristicCallbacks` and `NimBLEDescriptorCallbacks` methods now take `const NimBLEConnInfo&` instead of non-const.
* `NimBLECharacteristic::onNotify` callback removed as unnecessary, the library does not call notify without app input.
* `NimBLERemoteCharacteristic::getRemoteService` now returns a `const NimBLERemoteService*` instead of non-const.
* Add NimBLEUUID constructor that takes a reference to `ble_uuid_any_t`.
* `NimBLERemoteService::getCharacteristics` now returns a `const std::vector<NimBLERemoteCharacteristic*>&` instead of non-const `std::vector<NimBLERemoteCharacteristic*>*`
* `NimBLERemoteService::getValue` now returns `NimBLEAttValue` instead of `std::string`
* `NimBLEService::getCharacteristics` now returns a `const std::vector<NimBLECharacteristic*>&` instead of a copy of std::vector<NimBLECharacteristic *>.
* Remove const requirement for NimBLEConnInfo parameter in callbacks.
  Const is unnecessary as the data can't be changed by application code.
* Change NimBLERemoteCharacteristic::getRemoteService to return const pointer.

examples/basic/BLE_client/main/main.cpp

@@ -51,7 +51,7 @@ class MyClientCallback : public BLEClientCallbacks {
   }
 /***************** New - Security handled here ********************
 ****** Note: these are the same return values as defaults ********/
-  void onPassKeyEntry(const NimBLEConnInfo& connInfo){
+  void onPassKeyEntry(NimBLEConnInfo& connInfo){
       printf("Server Passkey Entry\n");
       /** This should prompt the user to enter the passkey displayed
        * on the peer device.
@@ -59,14 +59,14 @@ class MyClientCallback : public BLEClientCallbacks {
       NimBLEDevice::injectPassKey(connInfo, 123456);
   };
 
-  void onConfirmPIN(const NimBLEConnInfo& connInfo, uint32_t pass_key){
+  void onConfirmPIN(NimBLEConnInfo& connInfo, uint32_t pass_key){
       printf("The passkey YES/NO number: %" PRIu32 "\n", pass_key);
       /** Inject false if passkeys don't match. */
       NimBLEDevice::injectConfirmPIN(connInfo, true);
   };
 
   /** Pairing process complete, we can check the results in connInfo */
-  void onAuthenticationComplete(const NimBLEConnInfo& connInfo){
+  void onAuthenticationComplete(NimBLEConnInfo& connInfo){
       if(!connInfo.isEncrypted()) {
           printf("Encrypt connection failed - disconnecting\n");
           /** Find the client with the connection handle provided in desc */

examples/basic/BLE_notify/main/main.cpp

@@ -65,13 +65,13 @@ class MyServerCallbacks: public BLEServerCallbacks {
         return 123456;
     };
 
-    void onConfirmPIN(const NimBLEConnInfo& connInfo, uint32_t pass_key){
+    void onConfirmPIN(NimBLEConnInfo& connInfo, uint32_t pass_key){
         printf("The passkey YES/NO number: %" PRIu32 "\n", pass_key);
         /** Inject false if passkeys don't match. */
         NimBLEDevice::injectConfirmPIN(connInfo, true);
     };
 
-    void onAuthenticationComplete(const NimBLEConnInfo& connInfo){
+    void onAuthenticationComplete(NimBLEConnInfo& connInfo){
         /** Check that encryption was successful, if not we disconnect the client */
         if(!connInfo.isEncrypted()) {
             NimBLEDevice::getServer()->disconnect(connInfo.getConnHandle());

examples/basic/BLE_uart/main/main.cpp

@@ -67,13 +67,13 @@ class MyServerCallbacks: public BLEServerCallbacks {
         return 123456;
     };
 
-    void onConfirmPIN(const NimBLEConnInfo& connInfo, uint32_t pass_key){
+    void onConfirmPIN(NimBLEConnInfo& connInfo, uint32_t pass_key){
         printf("The passkey YES/NO number: %" PRIu32 "\n", pass_key);
         /** Inject false if passkeys don't match. */
         NimBLEDevice::injectConfirmPIN(connInfo, true);
     };
 
-    void onAuthenticationComplete(const NimBLEConnInfo& connInfo){
+    void onAuthenticationComplete(NimBLEConnInfo& connInfo){
         /** Check that encryption was successful, if not we disconnect the client */
         if(!connInfo.isEncrypted()) {
             NimBLEDevice::getServer()->disconnect(connInfo.getConnHandle());