Refactor NotificationRecipientService

What does this MR do?

It refactors NotificationRecipientService and Builders in order to make the code more readable.

It basically takes Builder classes and extracts them to separated files.

It also adds absolute basic specs for Builder::Default and NotificationRecipients::Service

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.