Make attenuator with enum filter values

[olliesilvester]

Fixes some of #927

This will require a change in mx_bluesky before merging

Instructions to reviewer on how to test:

1. Check changes are sensible
2. dodal connect p99 should still work assuming devices are connected
3. dodal connect i02_1 passes

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}

[olliesilvester]

Ugly, but maybe it has to be. Open to suggestions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.47%. Comparing base (75f1019) to head (df48e84).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #969      +/-   ##
==========================================
+ Coverage   95.88%   97.47%   +1.58%     
==========================================
  Files         145      147       +2     
  Lines        6240     6208      -32     
==========================================
+ Hits         5983     6051      +68     
+ Misses        257      157     -100     

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[DiamondJoseph]

Generally looks good, I have opinionated change requests.