Support relationship field data in REST responses

[nickcernis]

We had a request here to support returning relationship data in REST responses.

https://wordpress.org/support/topic/relationship-field-3/

Currently, relationship data is returned for WPGraphQL responses, but not REST:

> curl http://dev.local/wp-json/wp/v2/relationshiptests | jq
[
  {
    "id": 3937,
    "date": "2022-07-26T12:42:38",
    "date_gmt": "2022-07-26T12:42:38",
    "guid": {
      "rendered": "http://dev.local/?post_type=relationshiptest&p=3937"
    },
    "modified": "2022-07-26T12:42:38",
    "modified_gmt": "2022-07-26T12:42:38",
    "slug": "3937",
    "status": "publish",
    "type": "relationshiptest",
    "link": "http://dev.local/posts/relationshiptest/3937/",
    "title": {
      "rendered": "No Title 3937"
    },
    "author": 1,
    "template": "",
    "acm_fields": {
      "relationship": ""
    },
   …
  }
]

We would need to extend the REST resolver logic here:

atlas-content-modeler/includes/content-registration/custom-post-types-registration.php

Lines 131 to 134 in 8a2c543

	function handle_content_fields_for_rest_api( int $post_id, array $field, \WP_REST_Request $request ) {
	$meta_value = get_post_meta( $post_id, $field['slug'], true );
	switch ( $field['type'] ) {

To find and return related entry IDs, similar to the WPGraphQL resolver:

atlas-content-modeler/includes/content-registration/custom-post-types-registration.php

Lines 693 to 729 in 8a2c543

	'resolve' => static function ( Post $post, $args, $context, $info ) use ( $connection_args ) {
	$registry = \WPE\AtlasContentModeler\ContentConnect\Plugin::instance()->get_registry();
	$relationship = $registry->get_post_to_post_relationship(
	sanitize_key( $connection_args['to_slug'] ),
	sanitize_key( $connection_args['from_slug'] ),
	$connection_args['name']
	);
	if ( false === $relationship ) {
	return array();
	}
	$relationship_ids = $relationship->get_related_object_ids( $post->ID );
	if ( empty( $relationship_ids ) ) {
	return array();
	}
	$resolver = new PostObjectConnectionResolver(
	$post,
	$args,
	$context,
	$info
	);
	$resolver->set_query_arg(
	'post__in',
	$relationship_ids
	);
	if ( $connection_args['one_to_one'] ) {
	return $resolver->one_to_one()->get_connection();
	}
	return $resolver->get_connection();
	},