Make SMTP credentials optional

[dbutti]

Is your feature request related to a problem? Please describe.
When entering SMTP credentials for Defguard core under "Settings" in the web interface, username and password are compulsory. They should be optional instead, in order to make it possible to use an open SMTP relay.

Describe the solution you'd like
SMTP authentication should be optional

Describe alternatives you've considered
With the current version, only SMTP relays requiring authentication can be used.