Add resource links to JSON:API entrypoint

Thank you @brianperry → for you guidance in this.
Some inspiration from jsonapi_menu_items's sub-module: jsonapi_menu_items_hypermedia which help me to create a sub-module for this.

I reasoned that not everyone would need the `jsonapi_search_api` to always depend on the `jsonapi_hypermedia` module, so creating a sub-module made sense.

I'm changing the status of this to "Needs Review". If the module maintainers believe this is not necessary, they can move it back to "Postponed".

Oh yes I will need to add the test cases for the new sub-module, If the approach I have decided is accepted.

Performed an initial review and the code makes sense to me. Setting back to needs work so tests can be added.

I the test case. Not sure why it's all red :-(
I see other test also breaking.

I'm not sure what's causing the tests to fail for `Jsonapi_search_api_hypermedia` test file.

Jsonapi_search_api_hypermedia.Jsonapi_search_api_hypermedia
2	Jsonapi_search_api_hypermedia.Drupal\Tests\jsonapi_search_api_hypermedia\Functional\IndexHypermediaTest
✗	
Drupal\Tests\jsonapi_search_api_hypermedia\Functional\IndexHypermediaTest
exception: [Other] Line 0 of sites/default/files/simpletest/phpunit-2.xml:
PHPUnit Test failed to complete; Error: PHPUnit 8.5.31 by Sebastian Bergmann and contributors.

Testing Drupal\Tests\jsonapi_search_api_hypermedia\Functional\IndexHypermediaTest
E                                                                   1 / 1 (100%)R

Time: 1.8 seconds, Memory: 6.00 MB

There was 1 error:

1) Drupal\Tests\jsonapi_search_api_hypermedia\Functional\IndexHypermediaTest::testIndexHypermedia
Undefined index: jsonapi_hypermedia

/var/www/html/core/lib/Drupal/Core/Extension/ModuleInstaller.php:125
/var/www/html/core/lib/Drupal/Core/ProxyClass/Extension/ModuleInstaller.php:83
/var/www/html/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php:474
/var/www/html/core/tests/Drupal/Tests/BrowserTestBase.php:545
/var/www/html/core/tests/Drupal/Tests/BrowserTestBase.php:364
/var/www/html/modules/contrib/jsonapi_search_api/modules/jsonapi_search_api_hypermedia/tests/src/Functional/IndexHypermediaTest.php:46
/var/www/html/vendor/phpunit/phpunit/src/Framework/TestResult.php:703

--

There was 1 risky test:

1) Drupal\Tests\jsonapi_search_api_hypermedia\Functional\IndexHypermediaTest::testIndexHypermedia
This test did not perform any assertions

/var/www/html/core/tests/Drupal/Tests/Listeners/DrupalListener.php:127
/var/www/html/vendor/phpunit/phpunit/src/Framework/TestResult.php:436
/var/www/html/vendor/phpunit/phpunit/src/Util/PHP/AbstractPhpProcess.php:377
/var/www/html/vendor/phpunit/phpunit/src/Util/PHP/AbstractPhpProcess.php:187
/var/www/html/vendor/phpunit/phpunit/src/Framework/TestSuite.php:620
/var/www/html/vendor/phpunit/phpunit/src/TextUI/TestRunner.php:647
/var/www/html/vendor/phpunit/phpunit/src/TextUI/Command.php:235
/var/www/html/vendor/phpunit/phpunit/src/TextUI/Command.php:194

ERRORS!
Tests: 1, Assertions: 0, Errors: 1, Risky: 1.
✗	
Unknown
fail: [run-tests.sh check] Line 0 of :
FATAL Drupal\Tests\jsonapi_search_api_hypermedia\Functional\IndexHypermediaTest: test runner returned a non-zero error code (2).

I'm looking for some suggestion here.

It would also be nice to have the resource links in /admin/config/services/jsonapi/resource_types when using jsonapi_extras module. I don't know if it a feature that fits here or in the jsonapi_extras module. :)