Add rembg.com provider integration

Tom Krequested to merge
issue/removebg-3590688:3590688-rembg-provider into master

Closes #3590688

This merge request adds support for the rembg.com background removal API provider.

Changes include:

  • rembg.com provider integration
  • Provider switching support
  • Transparent PNG/WEBP processing
  • Improved image replacement handling
  • Provider-specific status messaging
  • Additional error handling and validation

The integration works with Drupal image styles and supports transparent background generation.

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