Support attribute-based plugin discovery

Note that this would need a core minimum of 10.2.

Thank you for such a well executed contribution!

I see that all tests passed in GitLab. Checking out the MR and running tests in DDEV via ddev-selenium-standalone-chrome:1.1.0 I get a couple of things:

HTML output was generated.
https://oauth2-client.ddev.site/sites/simpletest/browser_output/Drupal_Tests_oauth2_client_Functional_Oauth2ClientCollectionTest-93-40265769.html
https://oauth2-client.ddev.site/sites/simpletest/browser_output/Drupal_Tests_oauth2_client_Functional_Oauth2ClientCollectionTest-94-40265769.html
https://oauth2-client.ddev.site/sites/simpletest/browser_output/Drupal_Tests_oauth2_client_Functional_Oauth2ClientCollectionTest-95-40265769.html
https://oauth2-client.ddev.site/sites/simpletest/browser_output/Drupal_Tests_oauth2_client_Functional_Oauth2ClientUpdateTest-153-31269380.html


Time: 00:06.686, Memory: 6.00 MB

There was 1 failure:

1) Drupal\Tests\oauth2_client\Functional\Oauth2ClientUpdateTest::testOauth2UpdateHooks
The link Continue was not found on the page.
Failed asserting that an array has the key 0.

/var/www/html/docroot/core/tests/Drupal/Tests/UiHelperTrait.php:456
/var/www/html/docroot/core/tests/Drupal/Tests/UpdatePathTestTrait.php:50
/var/www/html/docroot/modules/oauth2_client/tests/src/Functional/Oauth2ClientUpdateTest.php:78

--

2 tests triggered 1 deprecation:

1) /var/www/html/vendor/symfony/error-handler/DebugClassLoader.php:341
Method "League\OAuth2\Client\OptionProvider\OptionProviderInterface::getAccessTokenOptions()" might add "array" as a native return type declaration in the future. Do the same in implementation "Drupal\oauth2_client\OAuth2\Client\OptionProvider\ClientCredentialsOptionProvider" now to avoid errors or add an explicit @return annotation to suppress this message.

Triggered by:

* Drupal\Tests\oauth2_client\Kernel\Oauth2ClientEntityKernelTest::testClientCredGetToken
  /var/www/html/docroot/modules/oauth2_client/tests/src/Kernel/Oauth2ClientEntityKernelTest.php:153

* Drupal\Tests\oauth2_client\Kernel\Oauth2ClientEntityKernelTest::testRefreshGetToken
  /var/www/html/docroot/modules/oauth2_client/tests/src/Kernel/Oauth2ClientEntityKernelTest.php:207

Investigating...

The failing test is was for the upgrade from 3.x to 4.0. Although there are still 150 or so installs on that version, it's not possible to upgrade from 3.x to the release that this code will land on as 3.x only runs in D9. Removed the test. Corrected other lint detected.