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 99.12%. Comparing base (a2e9629) to head (2b90e9b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1706   +/-   ##
=======================================
  Coverage   99.11%   99.12%           
=======================================
  Files         281      282    +1     
  Lines       10667    10689   +22     
=======================================
+ Hits        10573    10595   +22     
  Misses         94       94           

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