Add APIv4 MailingEventClickThrough

[colemanw]

Overview

Adds a missing API for tracking click-throughs in sent mailings.

It appears to be an oversight from #25059 which claimed to add it but didn't.

[civibot]

(Standard links)

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[eileenmcnaughton]

api\v4\Entity\ConformanceTest::testConformance with data set "MailingEventClickThrough" ('MailingEventClickThrough')
Civi\API\Exception\NotImplementedException: API (TrackableURL, get) does not exist (join the API team and implement it!)

/home/jenkins/bknix-dfl/build/build-0/web/sites/all/modules/civicrm/Civi/API/Request.php:51
/home/jenkins/bknix-dfl/build/build-0/web/sites/all/modules/civicrm/api/api.php:71
/home/jenkins/bknix-dfl/build/build-0/web/sites/all/modules/civicrm/Civi/Test/Api4TestTrait.php:292
/home/jenkins/bknix-dfl/build/build-0/web/sites/all/modules/civicrm/Civi/Test/Api4TestTrait.php:225
/home/jenkins/bknix-dfl/build/build-0/web/sites/all/modules/civicrm/Civi/Test/Api4TestTrait.php:127
/home/jenkins/bknix-dfl/build/build-0/web/sites/all/modules/civicrm/tests/phpunit/api/v4/Entity/ConformanceTest.php:285
/home/jenkins/bknix-dfl/build/build-0/web/sites/all/modules/civicrm/tests/phpunit/api/v4/Entity/ConformanceTest.php:152
/home/jenkins/bknix-dfl/extern/phpunit8/phpunit8.phar:1721

[colemanw]

Replaced by #26016