REST API: Add support for search_columns to the user endpoint.

[youknowriad]

This ticket tracks the backport of the changes introduced in ​https://github.com/WordPress/gutenberg/pull/67330

The idea is to add "search_columns" support to the REST API user controller in order to fix a Gutenberg bug in the post author selector.

A similar change has been introduced before for the posts controller in https://core.trac.wordpress.org/ticket/43867

Backports to change originally made in ​https://github.com/WordPress/gutenberg/pull/67330

This adds support to search_columns to the user controller, mirroring a change that was done in the past for the post controller.

Trac ticket: https://core.trac.wordpress.org/ticket/62596

Todo

• [ ] Add unit test.

@youknowriad, here's the patch.

diff --git tests/phpunit/tests/rest-api/rest-users-controller.php tests/phpunit/tests/rest-api/rest-users-controller.php
index 009200f401..fcaf7cdf5f 100644
--- tests/phpunit/tests/rest-api/rest-users-controller.php
+++ tests/phpunit/tests/rest-api/rest-users-controller.php
@@ -713,6 +713,9 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		$this->assertCount( 0, $response->get_data() );
 	}
 
+	/**
+	 * @ticket 62596
+	 */
 	public function test_get_items_search_columns() {
 		$request = new WP_REST_Request( 'GET', '/wp/v2/users' );
 		$request->set_param( 'search', 'yololololo' );
@@ -746,6 +749,27 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		$this->assertCount( 1, $response->get_data() );
 	}
 
+	/**
+	 * @ticket 62596
+	 */
+	public function test_get_items_seach_columns_without_permission() {
+		self::factory()->user->create(
+			array(
+				'display_name' => 'Adam',
+				'user_email'   => 'yololololo@example.localhost',
+			)
+		);
+
+		// Test user without sufficient capabilities - 'list_users'.
+		wp_set_current_user( self::$editor );
+
+		$request = new WP_REST_Request( 'GET', '/wp/v2/users' );
+		$request->set_param( 'search', 'yololololo' );
+		$request->set_param( 'search_columns', 'email' );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertCount( 0, $response->get_data() );
+	}
+
 	public function test_get_items_slug_query() {
 		wp_set_current_user( self::$user );

[joemcgill]

Planning to update the PHPUnit test on the PR and commit this.

I've pushed a commit to this PR that applies the PHPUnit test added by @Mamaduka (thanks!) and will commit this once confirming that all checks are passing.

Performance tests failing are fine. This is due to an unrelated change to those CI workflows that are failing due to this PR being more than 3 months old.

[joemcgill]

I missed the leading # before the ticket number in the commit message. This was fixed by [59892].

Merged in https://core.trac.wordpress.org/changeset/59892