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

[tomek-ac]

Hey @J0SUEFDZ could you please remove the version and changelog updates from this PR? We typically update the version in a separate task.

[tomek-ac]

I'd remove that change to lib/postmark/client.rb from this PR but other than that it looks good to me 👍

[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

[FrancescoK]

@J0SUEFDZ ever considered merging this?