feat(bitaxe): implement closed-loop fan control based on temperature

[rkuester]

Add automatic fan speed control that adjusts based on ASIC temperature. Currently the fan runs at 100% as a safe default.

See esp-miner's PID-based fan control implementation:
https://github.com/bitaxeorg/esp-miner/blob/66e4f1e63172f69a212d116ed8963fe36da62cb7/main/tasks/power_management_task.c#L109-L161