`howl`: fill out `nexus_structure` in the run start message

[rerpha]

I would like to be able to test any downstream consumers that rely on the nexus_structure field of a run start

this is loosely defined here but we need to write our own documentation on this when we start with our filewriter.

for now I think the easiest approach would be to take a json file which has a sensible default, load it and literally just use the contents of that file as the nexus_structure field. This default file will evolve over time but should probably contain some static data and a placeholder for the ev44 events which we also stream in howl