refactor: use omitzero for struct fields in API types

[kahirokunn]

Go 1.24 introduced the omitzero JSON tag option which properly handles struct fields. omitempty has no effect on non-pointer struct fields. the struct is never considered "empty" by encoding/json, so the tag is silently ignored. omitzero checks the zero value correctly for these types.

This commit replaces omitempty with omitzero on all non-pointer struct fields across the v1alpha1 API types (Ingress, Certificate, ClusterDomainClaim, ServerlessService and their List types).

Changes

• 🧹 Replace omitempty with omitzero on embedded ObjectMeta / ListMeta and top-level Spec / Status struct fields
• 🧹 Replace omitempty with omitzero on intstr.IntOrString field in HTTP01Challenge

/kind cleanup

Release Note

N/A

Docs

N/A

[knative-prow]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.33%. Comparing base (965e7c3) to head (74d8791).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1117   +/-   ##
=======================================
  Coverage   93.33%   93.33%           
=======================================
  Files          35       35           
  Lines        1005     1005           
=======================================
  Hits          938      938           
  Misses         53       53           
  Partials       14       14           

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[knative-prow]

@kahirokunn: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
build-tests_networking_main	74d8791	link	true	/test build-tests

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[dprotaso]

What problem are you trying to solve with omitzero ?

[kahirokunn]

There are no particular issues either.
I decided to try out omitzero because of the following review comments. However, since many third-party developers directly import and use the Go code, it would constitute a breaking change for them. Therefore, I concluded it would be better not to modify the released version, so I'm closing this for now.
#1116 (comment)