json schema for nexus_structure #118

We should write one of these so that both kafka_dae_control/saluki and kafka_file_writer can validate run start nexus_structures

as a first start this can allow basically any for module/config but longer term we might want to validate these options too.

also link to this from a comment in https://github.com/ISISComputingGroup/streaming-data-types/blob/master/schemas/pl72_run_start.fbs#L19

This should probably be in this repo (DataStreaming) along with any other json schemas which may be shared among software within the data streaming stack