WIP: add hx711 driver

[gen2thomas]

Add a driver for the weight cell chip HX711.

TODO:

• add example
• use functional interface according Add PinInput and PinOutput HAL #753
• introduce a PR for gobot as a reference implementation, see gpio(hx711): add driver for 24 bit ADC e.g. measure with load cells hybridgroup/gobot#1164
• try to switch from float32 to integer, at least for the exposed functions
• check finally the heap allocations
• check finally the conformance to https://tinygo.org/docs/guides/driver-design/

[gen2thomas]

this is not used internally, e.g. for blocking fast updates, but a caller could have a benefit if we add a "MaxCycleTime()" function to the drivers.Sensor interface