I19: update piezo device to expose the setpoint

[noemifrisina]

Updates access controlled piezo device to have a readable setpoint, ad they are not in a closed loop and the rbv is drifting a lot

Needed for i19-bluesky#166

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.16%. Comparing base (879440d) to head (024f28a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2165   +/-   ##
=======================================
  Coverage   99.16%   99.16%           
=======================================
  Files         353      353           
  Lines       13873    13874    +1     
=======================================
+ Hits        13757    13758    +1     
  Misses        116      116           

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