Extract NotificationRecipientService::Builder into separate files

A follow-up from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6626

Currently the various NotificationRecipientService::Builder are all defined in app/services/notification_recipient_service.rb.

• The code quality warning pops up.
• When setting up extension for these builders (for EE), the file name convention can be difficult.
• Not future proof if we keep adding new builders.

It would be nice to split those builders into separate files.