Search: Add a hook on returning filters. (jetpack:10706)

Summary:
 <!--- Provide a general summary of your changes in the Title above -->

 Fixes N/A(?)

 #### Changes proposed in this Pull Request:
 <!--- Explain what functional changes your PR includes -->

 I recently had a use case where I wanted to add custom filters (i.e. not
 a taxonomy, post type or date histogram) and found I could set the
 filter no problem but `get_filters()` would ignore it as it wasn't
 supported.

 I'd like to be able to easily support custom filters through a hook into
 `get_filters()`.

 #### Testing instructions:
 <!-- Please include detailed testing steps, explaining how to test your change. -->
 <!-- Bear in mind that context you working on is not obvious for everyone.  -->
 <!-- Adding "simple" configuration steps will help reviewers to get to your PR as quickly as possible. -->
 <!-- "Before / After" screenshots can also be very helpful when the change is visual. -->

 * Add custom filters using `set_filters()` e.g.

 ```php
 Jetpack_Search::instance()->set_filters(
 	[
 		'Price Range' => [
 			'type' => 'range',
 			'label' => 'price_range',
 			'field' => 'price',
 			'ranges' => $ranges,
 			'count' => count( $ranges ),
 		],
 	]
 );
 ```

 Call `get_filters()` and observe that the added (unsupported) filter is not available.

 #### Proposed changelog entry for your changes:
 <!-- Please do not leave this empty. If no changelog entry needed, state as such. -->

 * Added hook to `get_filters()` to allow the use of custom filters.

--

Automatically created by Jetpack Fusion from a Pull Request:
#10706

Test Plan: - [ ] Dummy test plan.

Reviewers: benoitperson, hypertextranch, gibrown, github-phab-bot

Tags: #touches_jetpack_files

Differential Revision: https://[private link]

Merges r185368-wpcom.

modules/search/class.jetpack-search.php

@@ -326,7 +326,7 @@ public function set_filters_from_widgets() {
 	 * @param WP_Query $query A WP_Query instance.
 	 */
 	public function maybe_add_post_type_as_var( WP_Query $query ) {
-		if ( $this->should_handle_query( $query ) && ! empty( $_GET['post_type'] ) ) {
+		if ( $query->is_main_query() && $query->is_search && ! empty( $_GET['post_type'] ) ) {
 			$post_types = ( is_string( $_GET['post_type'] ) && false !== strpos( $_GET['post_type'], ',' ) )
 				? $post_type = explode( ',', $_GET['post_type'] )
 				: (array) $_GET['post_type'];
@@ -460,7 +460,17 @@ public function search( array $es_args ) {
 	 * @return array Array of matching posts.
 	 */
 	public function filter__posts_pre_query( $posts, $query ) {
-		if ( ! $this->should_handle_query( $query ) ) {
+		/**
+		 * Determine whether a given WP_Query should be handled by ElasticSearch.
+		 *
+		 * @module search
+		 *
+		 * @since  5.6.0
+		 *
+		 * @param bool     $should_handle Should be handled by Jetpack Search.
+		 * @param WP_Query $query         The WP_Query object.
+		 */
+		if ( ! apply_filters( 'jetpack_search_should_handle_query', ( $query->is_main_query() && $query->is_search() ), $query ) ) {
 			return $posts;
 		}
 
@@ -515,7 +525,7 @@ public function filter__posts_pre_query( $posts, $query ) {
 	 * @param WP_Query $query The original WP_Query to use for the parameters of our search.
 	 */
 	public function do_search( WP_Query $query ) {
-		if ( ! $this->should_handle_query( $query ) ) {
+		if ( ! $query->is_main_query() || ! $query->is_search() ) {
 			return;
 		}
 
@@ -1830,27 +1840,6 @@ public function move_search_widgets_to_inactive( $module ) {
 		}
 	}
 
-	/**
-	 * Determine whether a given WP_Query should be handled by ElasticSearch.
-	 *
-	 * @param WP_Query $query The WP_Query object.
-	 *
-	 * @return bool
-	 */
-	public function should_handle_query( $query ) {
-		/**
-		 * Determine whether a given WP_Query should be handled by ElasticSearch.
-		 *
-		 * @module search
-		 *
-		 * @since  5.6.0
-		 *
-		 * @param bool     $should_handle Should be handled by Jetpack Search.
-		 * @param WP_Query $query         The WP_Query object.
-		 */
-		return apply_filters( 'jetpack_search_should_handle_query', $query->is_main_query() && $query->is_search(), $query );
-	}
-
 	/**
 	 * Transforms an array with fields name as keys and boosts as value into
 	 * shorthand "caret" format.