event_flatstore improvement: new header parameter

[nikbyte]

Summary

This PR adds per-file headers to the event_flatstore module.
With the new header parameter, you can write an arbitrary string (e.g. CSV column names) as the very first line of every newly created or rotated log file — making it easier to work with parsers and analysis tools.

Details

• Current behavior
  When a flatstore log file is rotated, the new file starts immediately with data lines — no header is added.

• Problem
  Many downstream tools expect a header row (e.g. in CSV files). Manually injecting headers after rotation is error-prone and inconvenient. Letting the module write the header automatically ensures consistency.

Solution

• New parameter

Parameter	Type	Default	Notes
header	string	(empty string)	Written as the first line of every new file. No trailing newline required. UTF-8 supported.

When the module opens a new file (initial, rotated, or reopened), it will prepend this string as a line if header is set.

Compatibility

• Fully backwards-compatible — if header is not configured, behavior remains unchanged.

[razvancrainea]

@nikbyte I don't like the idea of hardcoding one single header for any type of events - this will only make sense if you trigger just one single event through event_flatstore, but if you have two, that would be completely broken.
Perhaps there is a way of fetching the names of the fields that are triggered, at least for events with named fields, and dump them in the header. This information is available in the flat_raise function in the params argument.

[nikbyte]

@razvancrainea Thank you for your comment, I'll review it and try to find a better way.

[razvancrainea]

Hey @nikbyte ! Did you have time to look into a new approach for this? Thanks!

[nikbyte]

Hi @razvancrainea . I haven't had time to look at it yet, but I definitely have plans to redo it better way in the nearest future.