X509 User Authentication support using secrets reference

[marcschier]

This covers the implementation of X509 certificate based user authentication using secrets reference to a secret in file system or secrets storage. This should address the following past items filed by customers.

• User authentication via certificate #1208
• How to authenticate with a Certificate with Twin?  #385
• Error in twin service when using x509 authentication #121
• X509 4096-bit certificates as user credentials are too long #119

This issue covers the implementation and we will close the other 4 issues.