add update-only placeholder in nexus structure

I would like to be able to have a stream placeholder, in the same way as we use for events, in the `nexus_structure` field of a run start, which signifies that instead of appending to a time-series dataset, the writer should just overwrite the current scalar value.

Start by tracing how run-start nexus_structure values are handled and compare the existing event placeholder behavior. Done means a stream placeholder can identify update-only values so the writer overwrites the current scalar value instead of appending to a time-series dataset.

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