FastCS Eiger: Allow setting the photon energy and other MX settings

[DominicOram]

When arming the Eiger you need to provide the photon energy and a number of other metadata parameters (https://github.com/DiamondLightSource/dodal/blob/46132f0e066debaf906c1a80dcefca175b3f5c01/src/dodal/devices/eiger.py#L224). As part of #528 the signals were put in to do this but it still needs to be done as part of the arm of the eiger.

Acceptance Criteria

• When arming the eiger the metadata is set

[coretl]

Can we do this in the plan as bps.mv commands rather than putting this in arm? I was trying to restrict prepare to things that are common to all detectors, that are needed for the triggering system to know about, rather than expanding it to everything

[DominicOram]

Can we do this in the plan as bps.mv commands rather than putting this in arm? I was trying to restrict prepare to things that are common to all detectors, that are needed for the triggering system to know about, rather than expanding it to everything

See comment at #528 (comment).