Support for EP instant results custom proxy

Reimplements the custom proxy from https://github.com/10up/elasticpress-proxy

Currently WIP, still to do:

- [ ] Implement request signing for cloud environments
- [ ] Come up with a way to cache the db query results that can be cleared on index/reindex so query only occurs on each container once. Will need to delete cache on reindex if possible

Fixes #410

Feature was revealed for v11 so should be backported to v11-branch.

composer.json

@@ -17,7 +17,8 @@
 		"files": [
 			"inc/namespace.php",
 			"inc/analysis/namespace.php",
-			"inc/packages/namespace.php"
+			"inc/packages/namespace.php",
+			"inc/instant_results/namespace.php"
 		]
 	},
 	"extra": {

inc/instant_results/namespace.php

@@ -0,0 +1,51 @@
+<?php
+/**
+ * Altis Search Proxy for Instant Results.
+ *
+ * Based on the proxy plugin at https://github.com/10up/elasticpress-proxy
+ *
+ * @package altis/enhanced-search
+ */
+
+namespace Altis\Enhanced_Search\Instant_Results;
+
+use ElasticPress\Indexables;
+use ElasticPress\Utils;
+
+/**
+ * Bootstrap Instant Results Proxy.
+ *
+ * @return void
+ */
+function boostrap() {
+	add_filter( 'ep_instant_results_available', '__return_true' );
+	add_action( 'ep_instant_results_template_saved', __NAMESPACE__ . '\\save_template' );
+	add_filter( 'ep_instant_results_search_endpoint', __NAMESPACE__ . '\\set_proxy' );
+}
+
+/**
+ * Save the a PHP file with the search query template and the post index URL into the uploads folder.
+ *
+ * @param string $search_template The search template.
+ * @return void
+ */
+function save_template( string $search_template ) : void {
+	$post_index_base = trailingslashit( Utils\get_host( true ) );
+	foreach( Utils\get_sites() as $site ) {
+		$post_index = Indexables::factory()->get( 'post' )->get_index_name( $site['blog_id'] );
+		update_site_option( "ep_{$site['blog_id']}_post_index_url", $post_index_base . $post_index, false );
+		update_site_option( "ep_{$site['blog_id']}_instant_results_template", $search_template, false );
+	}
+}
+
+/**
+ * Set the custom proxy as the search endpoint.
+ *
+ * @return string
+ */
+function set_proxy() : string {
+	return sprintf( '%sproxy.php?blog_id=%d',
+		plugin_dir_url( __FILE__ ),
+		get_current_blog_id()
+	);
+}