FastCS Eiger: Pass datatype from eiger to odin

[DominicOram]

Following #528 we should continue implementing the full functionality for the eiger that we need in MX. The datatype of data coming out of the eiger is dependant on the exposure time set on the detector. Odin needs to know the datatype up front so that it can correctly write data. During arm/open we need to get this out of the eiger and put it into odin. e.g. see https://github.com/DiamondLightSource/dodal/blob/46132f0e066debaf906c1a80dcefca175b3f5c01/src/dodal/devices/eiger.py#L320

Note that we need to do this after the exposure time has been accepted by the eiger as this is when it calculates the new bit depth

Acceptance Criteria

• When the fastcs eiger is armed it correctly forwards the bit depth to odin

[DominicOram]

@coretl is going to discuss with @GDYendell if we can do this in odin. Otherwise the plan would be to set exposure time and then datatype as the first thing in the plan for the detector (meaning that it won't be in parallel so we will have to see if this hurts performance)