Prevent selection of unsupported processors

Problem/Motivation

Currently we wait until the \Drupal\avif\Plugin\AvifProcessor\ImageMagick::convert method to test if the processor supports AVIF, and similarly for GD in ✨ Add support for GD Needs review .

Instead we should prevent these processors from being selected if AVIF is not supported.

Steps to reproduce

Proposed resolution

• Validate the configuration form to check if the selected processor is valid
• Add a requirements check for the currently configured processor

Remaining tasks

User interface changes

API changes

Data model changes