PHPMailer fork of DKIM validator

This is a major overhaul of the PHP DKIM validator created by @angrychimp and @bostjan.

• Switched to the PSR-12 coding standard
• Restructured classes with PHPMailer\DKIMValidator namespace.
• Numerous bug fixes
• Lots of new validations on many of the DKIM header and public key fields
• Requires PHP 7.2 or later