Create helper functions for making soft signals

[DominicOram]

Creating a soft signal is a bit long winded, we should make helper functions like https://github.com/DiamondLightSource/dodal/blob/main/src/dodal/devices/ophyd_async_utils.py

Acceptance criteria

• There are helper functions in ophy-async for making soft signals
• We remove the ones in dodal

[callumforrester]

Reviewers: @DominicOram @coretl @mrakitin

[jsouter]

See #217, as I say, can be moved if it's not core enough to be in core.signal. Will also make the equivalent import change in Dodal once I know the version number for ophyd_async with this change.