PM-1743. update Postmark Ruby gem to support CustomTracking

[J0SUEFDZ]

JIRA

• https://activecampaign.atlassian.net/browse/PM-1734

What

• Adds support for CustomTracking in the postmark gem.
• Since the gem directly communicates with the API, all responses already include the changes to support CustomTrackingCNAMEValue, CustomTrackingDomain, and CustomTrackingVerified values where required, so no extra work is needed.
• This PR enables the possibility to verify the domain using verify_domain_custom_tracking

Why

With CustomTracking almost ready to ship to production, we needed to add support for CustomTracking in this gem.

Tests

Manual tests for all endpoints pertinent to Custom Tracking, specified in the JIRA ticket, are returning the expected outut.

[J0SUEFDZ]

FYI Tried to ask for approval from the postmark-ruby team but it doesn't appear on the list, so I asked for a review individually

[J0SUEFDZ]

Thanks @tomek-ac. From my understanding, I can merge this without releasing, but I'll wait until the API is released to merge and release this. I'll follow the RELEASE.md instructions when I do it