Implement SSL API

[FrontEndCoffee]

Documentation: https://dm.realtimeregister.com/docs/api/ssl

Module:

• Add a SslApi module in the Api namespace
• Add this module to the RealtimeRegister class.

Endpoints:

• Get certificate
• List certificates
• Download certificate
• List DCV e-mail addresses
• Resend DCV
• Get product
• List products
• Request certificate
• Renew certificate
• Reissue certificate
• Revoke certificate
• Send subscriber agreement
• Add note
• Schedule validation call

When implementing the endpoints, make sure to include links to the documentation using the @see annotation in docblocks.

Tests:

• Add tests for all the added domain objects
• Add tests for all the added endpoints

[zbrag]

This has been done in #81 and #84 (Which is still open at the moment)