[Add] AR51 readout data (de)serialiser

[g5t]

There is no deserialiser for AR51 readout data for the *_detector_samples Kafka topic(s).

This PR adds a serialiser and deserialiser, but actually using these will require understanding the underlying detector data format and converting the deserialised data (typically to a Numpy structured array, using frombuffer not unlike fromfile used here)

A possible improvement could be a deserialiser that takes the structured array datatypes and does the conversion directly, but this is not strictly part of the existing flatbuffer schema so not included at the moment.