feat(mail): refactor email service loading and docs

[mariogarcia]

Before getting started

Update the new email schema configuration from application.yml.template. Email configuration structure has changed.

Testing steps

• check tests passed
• check removing all email configuration. Scheduling service should still work and print out some traces
• check adding only email related configuration only
• check changing email -> disabled configuration only. You should see DummyEmailService logs
• add aws credential information. Now scheduling service should be using AWS email service.
• check changing email -> disabled and see how AWS service works in disabled mode
• High five!