Add filters to allow modifying the plugin data

[khromov]

👋 After using the plugin for a headless project I found myself in need to extend the block data.

For example, I needed to add the image dimensions to a core/image block. The width/height isn't present in the attrs array unless the user enters it using Gutenberg manually. With the new hook rest_api_blocks_handle_do_block you can prefill this data from the WordPress database if the user hasn't set it:

add_filter('rest_api_blocks_handle_do_block', function($block) {
    if($block['blockName'] === 'core/image') {
        $metadata = wp_get_attachment_metadata($block['attrs']['id']);

        if(!($block['attrs']['width'] ?? false) && $metadata['width']) {
            $block['attrs']['width'] = $metadata['width'] . 'px';
        }

        if(!($block['attrs']['height'] ?? false) && $metadata['height']) {
            $block['attrs']['height'] = $metadata['height'] . 'px';
        }
    }

    return $block;
});

Also added a couple of more hooks for completion.