Block Editor: Expose api_version in the block type and the REST endpoint.

The new block editor included in 5.6 introduces an api_version property
that indicates which block API version the block is using. 
This commits makes this property available on the block type and the endpoint.

Props TimothyBlynJacobs, gziolo.
Fixes #51529.



git-svn-id: https://develop.svn.wordpress.org/trunk@49224 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/blocks.php

@@ -207,6 +207,7 @@ function register_block_type_from_metadata( $file_or_folder, $args = array() ) {
 		'supports'        => 'supports',
 		'styles'          => 'styles',
 		'example'         => 'example',
+		'apiVersion'      => 'api_version',
 	);
 
 	foreach ( $property_mappings as $key => $mapped_key ) {