[BREAKING] Refactor attributes

[h2zero]

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.
• 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 *>.