Use Random Images from Library for Block Patterns

[jauyong]

Feature description

I want to display block patterns with images so that editors can get inspired of great design with the block patterns

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Acceptance criteria

1. After adding a block pattern with an image, use a random image from the media library
2. For block patterns with multiple images, avoid using duplicate images if there are enough images available in the library

Implementation brief

QA testing instructions

Demo

Changelog entry