Move EntityToJsonApi service to JSON API Extras

#2958504: Stop using jsonapi.entity.to_jsonapi service in ResourceTestBase made this easy peasy :)

We already have consensus about this among the module maintainers, core maintainers are asking for this, and patch passes tests locally. I'll be bold here for a moment 🤞

Darn! So close :D

+++ b/tests/src/Functional/ResourceTestBase.php
@@ -181,11 +181,8 @@ abstract class ResourceTestBase extends BrowserTestBase {
-    // Create an account, which tests will use. Also ensure the @current_user
-    // service uses this account, to ensure the @jsonapi.entity.to_jsonapi
-    // service that we use to generate expectations matching that of this user.
+    // Create an account, which tests will use.
     $this->account = $this->createUser();
-    $this->container->get('current_user')->setAccount($this->account);

This is the only change that could possibly cause those failures. Confirmed locally that it fixes the failures.

Created CR: https://www.drupal.org/node/2983182.