Data streaming: run starts/stops

Run starts and stops will be dealt with by kafka_dae_control and the flatbuffers blobs will be constructed in this process. It may need to be hooked onto by ISISDAE for older instruments using DAE2/DAE3 and the ISISICP.

Run starts will contain static and streamed data in the nexus_structure, including things like run_number, instrument_name and so on which will get written to a file.

Run starts will also contain metadata used by kafka_dae_diagnostics, in a json schema defined by https://github.com/ISISComputingGroup/DataStreaming/issues/29 - this is so kafka_dae_diagnostics does not have to try and parse the nexus_structure as it doesn’t know or care about NeXus files.