[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.

src/NimBLEAttValue.h

@@ -192,12 +192,7 @@ class NimBLEAttValue {
         return setValue(reinterpret_cast<const uint8_t*>(s), len);
     }
 
-    /**
-     * @brief Get a pointer to the value buffer with timestamp.
-     * @param[in] timestamp A pointer to a time_t variable to store the timestamp.
-     * @returns A pointer to the internal value buffer.
-     */
-    const uint8_t* getValue(time_t* timestamp = nullptr) const {
+    const NimBLEAttValue& getValue(time_t* timestamp = nullptr) const {
         if (timestamp != nullptr) {
 # if CONFIG_NIMBLE_CPP_ATT_VALUE_TIMESTAMP_ENABLED
             *timestamp = m_timestamp;
@@ -205,7 +200,7 @@ class NimBLEAttValue {
             *timestamp = 0;
 # endif
         }
-        return m_attr_value;
+        return *this;
     }
 
     /**
@@ -271,7 +266,15 @@ class NimBLEAttValue {
         if (!skipSizeCheck && size() < sizeof(T)) {
             return T();
         }
-        return *(reinterpret_cast<const T*>(getValue(timestamp)));
+        if (timestamp != nullptr) {
+# if CONFIG_NIMBLE_CPP_ATT_VALUE_TIMESTAMP_ENABLED
+            *timestamp = m_timestamp;
+# else
+            *timestamp = 0;
+# endif
+        }
+
+        return *(reinterpret_cast<const T*>(m_attr_value));
     }
 
     /*********************** Operators ************************/