feat: add support for custom tracking domains

[christina-de-martinez]

Summary by cubic

Add custom tracking domains and tracking controls to domain APIs and webhooks. You can set a tracking subdomain and enable/disable click and open tracking, plus manage sending/receiving capabilities.

• New Features

  • Domain models (Domain, DomainEventData) now include open_tracking, click_tracking, tracking_subdomain, and capabilities.
  • DomainAddData: add open_tracking, click_tracking, tracking_subdomain, tls, and capabilities.
  • DomainUpdateData: add capabilities and tracking_subdomain (omitted when null).
  • New DomainCapabilities with sending and receiving.
  • DomainRecord examples now include Tracking.
  • Tests cover JSON (de)serialization for capabilities and tracking.

• Migration

  • Set DNS for the tracking subdomain (e.g., CNAME for links.example.com) and re-verify the domain after changes.
  • tracking_subdomain cannot be cleared once set; updates require DNS re-verification.

^{Written for commit 8f8ea06. Summary will update on new commits.}

[cubic-dev-ai]

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