Implement the `isThreadConnected` method

[SuGlider]

@SuGlider With availability of Matter over Thread, could we implement the isThreadConnected method? Probably using chip::DeviceLayer::ConnectivityMgr().IsThreadAttached()?

arduino-esp32/libraries/Matter/src/Matter.cpp

Lines 178 to 182 in fc8ce8f

	#if CHIP_DEVICE_CONFIG_ENABLE_THREAD
	bool ArduinoMatter::isThreadConnected() {
	return false; // Thread Network TBD
	}
	#endif

Originally posted by @loozhengyuan in #11965 (comment)