Add NF support for optional Certificate Authentication for HTTPS.

PROPOSED REQUIREMENT:  The VNF or PNF MUST support one of the following authentication methods for authenticating HTTPS connections to the DCAE VES Event Listener:

• The preferred method is Certificate Authentication
• The non-preferred option is Basic Authentication.

REASON: Add NF support for Certificate Authentication for HTTPS.  Certificates are more secure and more scalable.  Certificate Authentication is preferred for HTTPS.  Basic Auth is still supported if an NF can not support certificates.