Expose serialized template content on WP_Block_Template.

When building the template, initialize WP_Block_Template with the serialized block content so callbacks registered to the `hooked_block_types` filter can access the content.

src/wp-includes/block-template-utils.php

@@ -532,6 +532,7 @@ function _build_block_template_result_from_file( $template_file, $template_type
 	$template->has_theme_file = true;
 	$template->is_custom      = true;
 	$template->modified       = null;
+	$template->content        = $template_content;
 
 	if ( 'wp_template' === $template_type && isset( $default_template_types[ $template_file['slug'] ] ) ) {
 		$template->description = $default_template_types[ $template_file['slug'] ]['description'];