Add Support for c=relaxed/relaxed DKIM Canonicalization

Description:
Currently, Kerio Connect hardcodes DKIM generation to relaxed/simple. This lack of custom configuration causes outbound email authentication to fail if downstream spam filters or intermediate relays modify whitespace or alter parts of the email body text.

Requested Change:
Please update the DKIM signature settings (either via the Web Admin Console or a configuration file toggle) to support c=relaxed/relaxed canonicalization. This will ensure robust DMARC compliance and align Kerio Connect with modern delivery standards used by major providers like Microsoft 365 and Google Workspace.