Add post_types field to the template REST API response

We want to use post_type for filtering email templates.
There is a PR that adds this property https://github.com/WordPress/wordpress-develop/pull/7530
but it is not merged.
This commit added the property and also future compatibility check
so that we don't attempt to add it after the PR is merged.
[MAILPOET-6356]

packages/js/email-editor/src/store/selectors.ts

@@ -248,10 +248,9 @@ export const getEmailTemplates = createRegistrySelector(
 			} )
 			// We still need to filter the templates because, in some cases, the API also returns custom templates
 			// ignoring the post_type filter in the query
-			?.filter(
-				( template ) =>
-					// @ts-expect-error Missing property in type
-					template.plugin === 'mailpoet'
+			?.filter( ( template ) =>
+				// @ts-expect-error Missing property in type
+				template.post_types.includes( 'mailpoet_email' )
 			)
 );