There are many cases where we need a derived signal (see bluesky/ophyd-async#525). In the absence of one in ophyd-async it is preferable that we try and keep the places where we're using derived signals going through one code path in dodal. This is so that the migration to the standard solution is easier.

Currently we have create_hardware_backed_soft_signal which creates a read-only signal but there are cases where the derived signal should be read/write. It is very easy to change create_hardware_backed_soft_signal to do this so it is better to do this now, and keep everyone using the same style/code path, than to wait on bluesky/ophyd-async#525.

Acceptance Criteria

• create_hardware_backed_soft_signal creates a read/write signal that uses a provided set method for the writing.