Provide a way to override liveblog pusher credentials

[rthideaway]

Currently the following is not possible:

$config['liveblog.notification_channel.liveblog_pusher'] = [
  'app_id' => $overriden_pusher_app_id,
  'key' => $overriden_pusher_key,
  'secret' => $overriden_pusher_secret,
];

It's very important for us to not store sensitive data in repository so we would like to use config overrides instead in settings.php. So content of liveblog.notification_channel.liveblog_pusher.yml will be something symbolic like:

app_id: 'app_id_placeholder'
key: 'key_placeholder'
secret: 'secret_placeholder'

And then we override pusher credentials by variables, which are injected to settings.php and are not part of project repository. For this we need the pusher to provide a way to override config. With current set up it's not possible since config variables are loaded from yaml only and ignoring config overrides in settings.php

[rthideaway]

Currently I am using the following quick patch to get config overrides working. https://www.drupal.org/files/issues/2874796-2-liveblog-let-config-override.patch