Adds ability to configure the Registry notification_secret

What does this MR do?

• This is primarily utilized for Geo
• We create the ability to configure our Rails application with the secret item required
• This adds the necessary notification configuration for the container registry
• Adds the necessary testing with a helper to dig deep into the nesting produced by our YAML files
• Logic added to ensure that when geo is enabled, any existing notifiers are merged with the new geo_event notifier
• Due to #2153 we'll create a native secret as a string in an array
  • But also due to the same, we include a bit of awkward logic to peel the string out of the array when populating it into the gitlab.yml config file
• Adds a hint of documentation related to some feature being taken advantage of to send data from a template into a variable for later consumption
• Proposal adds the following:

global:
  geo:
    registry:
      syncEnabled: true

The above will configure a secret with a sane default if required and set this on the gitlab.yml file and the notifications for geo_event in the Container Registry

As this gitlab.yml file is shared among all deployments, this secret needs to be mounted into the init containers for Sidekiq and Webservices

Related issues

Closes: #2142 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA - n/a
• Equivalent MR/issue for omnibus-gitlab opened n/a