EN-144: Add custom tracking domain support

[nylas-it-ops]

Summary

• add optional TrackingOptions.domainName support, serialized as tracking_options.domain_name
• add a fluent TrackingOptions.Builder and preserve the existing four-argument Java constructor
• cover supplied and omitted values with model and request serialization tests
• add a runnable Java example for regular sends, drafts, scheduled sends, and Transactional Send
• document the distinction between the Transactional sender domain and the tracking hostname

Impact

Java and Kotlin callers can select an active organization-owned custom hostname for link and open tracking. Requests that omit domainName keep their existing JSON shape and default tracking behavior.

Validation

• ./gradlew test :examples:compileJava
• ./gradlew lintKotlin
• inspected the compiled TrackingOptions API to confirm getDomainName() and the legacy four-argument constructor are present

No live API or staging send was performed.

License

I confirm that this contribution is made under the terms of the MIT license and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.73%. Comparing base (d54e4d2) to head (c1c439a).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #336   +/-   ##
=========================================
  Coverage     83.73%   83.73%           
  Complexity      486      486           
=========================================
  Files            44       44           
  Lines          1359     1359           
  Branches        115      115           
=========================================
  Hits           1138     1138           
  Misses          149      149           
  Partials         72       72           

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