Refactor web UI config item C++ implementation #1325

Job	Run time
build (examples/minimal_app.cpp, esp32dev)	1m 28s
build (examples/analog_input.cpp, esp32dev)	1m 34s
build (examples/hysteresis.cpp, esp32dev)	1m 28s
build (examples/lambda_transform.cpp, esp32dev)	1m 35s
build (examples/manual_networking.cpp, esp32dev)	1m 34s
build (examples/minimal_app.cpp, esp32dev)	1m 31s
build (examples/rpm_counter.cpp, esp32dev)	1m 43s
build (examples/async_repeat_sensor.cpp, esp32dev)	1m 30s
build (examples/repeat_sensor_analog_input.cpp, esp32dev)	1m 28s
build (examples/freertos_tasks.cpp, esp32dev)	1m 34s
build (examples/raw_json.cpp, esp32dev)	1m 33s
build (examples/constant_sensor.cpp, esp32dev)	1m 28s
build (examples/time_counter.cpp, esp32dev)	1m 29s
	19m 55s

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