Introduce Remote Dev Settings module

Issue: Introduce Remote Dev Settings module (#442103 - closed)

What does this MR do and why?

Implements changes described in Introduce Remote Dev Settings module (#442103 - closed)

MR acceptance checklist

See #442103 (closed)

How to set up and validate locally

• Verify overriding of all supported values from ee/lib/remote_development/settings/defaults_initializer.rb via ENV vars in the form GITLAB_REMOTE_DEVELOPMENT_*
• Verify overriding of default_branch_name in application settings. Since this is not used directly, you can test by displaying it:
  • Change default branch name in http://gdk.test:3000/admin/application_settings/repository
  • Open bin/rails c console
  • Run RemoteDevelopment::Settings.get_single_setting(:default_branch_name)
  • Verify it uses the overridden value from current settings
• Verify precedence that ENV vars override current settings:
  • With the current setting for default branch still overridden, set an ENV var override to a different value in GITLAB_REMOTE_DEVELOPMENT_DEFAULT_BRANCH_NAME
  • Start a new bin/rails c and verify the ENV var value is used.