nexus-writer: respect `nexus_structure` only for `f144` placeholders

[rerpha]

This is a start on making the nexus writer respect the nexus_structure field of a pl72_run_start blob.

This will mean that instead of collecting everything from the sample env or run logs topic, the stream placeholders set up a big list of "interesting" PVs, which at run start you may as well create a group + datasets for. Probably also on run start you want to go back in time and get the most recent value (this is reliant on ISISComputingGroup/EPICS-inst_servers#445 as the update will need to be regular and less than the retention rate of the topic)

I think this will probably be the most important part of the nexus_structure work as events are in a regular place detector_1_events and it means we can fill in lots of other stuff outside selog with epics value updates ie. the list of users and so on (though we'll need to make sure we forward them!) though this depends on strings being supported (nexus format actually needs to change for this) and a decided mechanism on a stream placeholder which isn't time-series aka just gets overwritten if there's an update. Covered in #424