[QA] T86359: Updating jenkins for extension registration changes

[Legoktm]

Hi,

We're currently working on converting[1] all WMF-deployed extensions to
use the new extension registration[2] system that was introduced in 1.25.

I filed T86359[3] to track the updates we need to make to jenkins to
accommodate the new system:
* Use wfLoadExtensions() instead of require_once if an extension.json
file is present
 * I submitted a patch[4] for this, how can I help in getting it deployed?
* Run validateRegistrationFile.php whenever a extension/skin.json is
modified (similar to the php-composer-validate job).
 * This depends upon an external library, what would be the best way to
implement this?

[1] https://phabricator.wikimedia.org/T87875
[2] https://www.mediawiki.org/wiki/Extension_registration
[3] https://phabricator.wikimedia.org/T86359
[4] https://gerrit.wikimedia.org/r/184003

-- Legoktm