Add the ability to choose what preview provider use in priority when multiples are available for the same filetype.

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Is your feature request related to a problem? Please describe.
Imagick handle better HEIC preview than Imaginary, but Imaginary supports PDFs and Illustrator files preview, which Imagick doesn't supports.
But, we can't enable both Imaginary AND Imagick HEIC preview, and tell nextcloud to use Imagick in priority. It always uses Imaginary if available.

Describe the solution you'd like
Add way to decides which preview provider should be used in priority. This could be determined by the order of previews providers in the config list.

Describe alternatives you've considered
Not any.

Additional context
Imagick handles better HEIC orientations than Imaginary.

Review the preview-provider configuration and the existing selection behavior for Imagick and Imaginary. Define how provider order expresses priority, while preserving each provider’s supported file types, and verify that HEIC, PDF, and Illustrator previews follow the configured order.