Split up fastcs eiger plan

[jacob720]

Fixes #1076

Required by DiamondLightSource/mx-bluesky#1436

Instructions to reviewer on how to test:

1. Do thing x
2. Confirm thing y happens

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 99.09%. Comparing base (400fe84) to head (c65bc9a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1706   +/-   ##
=======================================
  Coverage   99.09%   99.09%           
=======================================
  Files         276      276           
  Lines       10147    10153    +6     
=======================================
+ Hits        10055    10061    +6     
  Misses         92       92           

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