Add open signal to FastShutter

[oliwenmandiamond]

Fixes #1985

Instructions to reviewer on how to test:

1. Check implementation make sense
2. Check tests pass

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 99.11%. Comparing base (a039d73) to head (05bd592).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1987   +/-   ##
=======================================
  Coverage   99.11%   99.11%           
=======================================
  Files         319      319           
  Lines       12267    12279   +12     
=======================================
+ Hits        12158    12170   +12     
  Misses        109      109           

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[jacob720]

Looks good