Finishes the implementation of the stubbed NotificationServiceClient.
Sends push notification request messages to the endpoint configured in
the EchoPushServiceBaseUrl setting. For example, to send messages to an
instance of mediawiki/services/push-notifications running on the host
machine while running MediaWiki in Docker, add the following line to
LocalSettings.php:
$wgEchoPushServiceBaseUrl = 'http://172.17.0.1:8900/v1/message';
Bug: T252899
Change-Id: Icab7825e9080d6b1a4cfc5e12fed1da221ce4610
