Update to ophyd_async 0.3a2

[DominicOram]

We should move to the latest ophyd_async. There are a number of things this breaks and improvements we can easily make as part of it:

• PandAs in ophyd-async 0.3 require DirectoryProvider information #466
• Update all references to motor fields that may have changed in Breaking Change: Rename and add motor fields bluesky/ophyd-async#174
• Replace our epics_signal_rw_rbv with the one written in Add SignalRW helpers bluesky/ophyd-async#221 ( Use ophyd_async RBV helper #446)
• Use the new functions for soft signals Add create_soft_signal_r and creates_soft_signal_rw methods bluesky/ophyd-async#217 Moved to Use new ophyd_async soft signal helpers  #482