B21: Add device for the flame spectrometer

To run collections with the flame spectrometer (https://github.com/DiamondLightSource/fastcs-flame) B21 needs an associated ophyd-async device.

For now it can just be a set of signals, it doesn't need to be a standard detector (though it should be at some point)

## Acceptance Criteria
* There is an ophyd-async device that can be used to take data with the flame spectrometer