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.