Python script to export extra fields of items in a specific resource category

Enhance resource_extra_fields_export example script to support
multiple API response formats and improve robustness.

- Use `_preload_content=False` and manually decode JSON response.
- Support metadata returned as either JSON string or object.
- Normalize extra field keys for case/space-insensitive matching.
- Add backward compatibility for different elabapi_python versions
  (dict-style and object-style access).
- Use safe `.get()` access for item fields.
- Ignore generated export files via `.gitignore` (export.*).

This makes the example script more resilient across API versions
and metadata formats.

Co-authored-by: Mouss <104678778+MoustaphaCamara@users.noreply.github.com>

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