Make it easy to make a hardware backed soft signal

[DominicOram]

Often the soft signals on a device may be derived from the value of real hardware signals. As such, when the soft signal is read it should read the real hardware and provide its value based on this. This can be done by having the device optionally provide a readback function when creating the signal.

See DiamondLightSource/dodal#782 for more detail on the usecase and DiamondLightSource/dodal#789 for a solution.

[Tom-Willemsen]

This sounds quite similar to #525 , maybe good to add your use-case to that issue as well?

[DominicOram]

Thanks, I agree, closing this as a duplicate.