Add --ghostscript-jpeg-quality and --ghostscript-jpeg-maxdpi

Expose Ghostscript's -dJPEGQ and image downsampling switches as
advanced, plugin-scoped options for tuning PDF/A output, without
polluting the central OcrOptions registry. The optimizer's existing
--jpeg-quality remains the recommended JPEG quality control.

- GhostscriptOptions gains jpeg_quality and jpeg_maxdpi fields and CLI
  args (advanced help text). jpeg_quality=0 is honored as Ghostscript's
  maximum compression rather than being silently coerced to the default.
- _exec.ghostscript.generate_pdfa() forwards both values; when
  jpeg_maxdpi is set, downsample threshold is pinned at 1.0.
- _get_plugin_options falls back to extra_attrs for namespaced fields
  so plugins can own their options without registering them centrally.
- Documentation explains the rationale: Ghostscript is the legacy path
  (pypdfium + verapdf is preferred in v17+), the optimizer is the
  supported file-size lever, and lowering quality is almost always a
  better trade than downsampling.

docs/cookbook.md

@@ -31,6 +31,16 @@ ocrmypdf --output-type pdf input.pdf output.pdf
 ocrmypdf --output-type pdfa --pdfa-image-compression jpeg input.pdf output.pdf
 ```
 
+### Reduce JPEG quality with the optimizer
+
+This is the recommended way to shrink JPEG content in the output. The
+optimizer applies regardless of `--output-type`, so it works on both
+plain PDFs and Ghostscript-produced PDF/A files.
+
+```bash
+ocrmypdf --optimize 2 --jpeg-quality 60 input.pdf output.pdf
+```
+
 ### Modify a file in place
 
 The file will only be overwritten if OCRmyPDF is successful.