Parse existing diagnostic.log files

The support diagnostic includes a `diagnostic.log` file tracking the progress of the diagnostic collection tool itself. This can be useful when troubleshooting collection issues and is only a few hundred lines at most.

Add a parser/processor to forward these entries into the `logs-diagnostic-esdiag` data stream.

Related to #320

Start by tracing how existing diagnostic files are parsed and how entries are forwarded into data streams. Use diagnostic.log and the logs-diagnostic-esdiag data stream as the scope; the work is done when its entries are parsed and forwarded there.

Written by the indexing model from the issue text.