[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/NimBLEDevice.h

@@ -31,7 +31,7 @@
 #endif
 
 #if defined(CONFIG_BT_NIMBLE_ROLE_CENTRAL)
-#include "NimBLEClient.h"
+class NimBLEClient;
 #endif
 
 #if defined(CONFIG_BT_NIMBLE_ROLE_PERIPHERAL)
@@ -46,6 +46,7 @@
 #endif
 
 #include <string>
+#include <array>
 
 #define BLEDevice                       NimBLEDevice
 #define BLEClient                       NimBLEClient
@@ -236,6 +237,12 @@ private:
 #endif
 };
 
+#if defined( CONFIG_BT_NIMBLE_ROLE_CENTRAL)
+#include "NimBLEClient.h"
+#include "NimBLERemoteService.h"
+#include "NimBLERemoteCharacteristic.h"
+#include "NimBLERemoteDescriptor.h"
+#endif
 
 #endif // CONFIG_BT_ENABLED
 #endif // MAIN_NIMBLEDEVICE_H_