feature: Add support for omitzero struct tag

[fxamacker]

Go 1.24 introduced the omitzero struct tag and it should be added here before fxamacker/cbor v2.8.0.

I'm taking some of my postponed Christmas break this week (March 24-28), so I can maybe add this feature during break.

[liggitt]

ha... I literally just started looking into this as part of https://github.com/kubernetes/kubernetes/pull/130989/commits ... happy to help contribute a constrained version of that here (maybe just when building with go 1.24, maybe just when falling back to using the json tag options on a struct)

[fxamacker]

@liggitt Amazing timing!

Please feel free to open a PR to add this feature. 🙏

For this library, it is OK to constrain (or not constrain) to 1.24. Either way works for me, with a slight preference for not constraining because it may simplify maintenance and testing.

BTW, fxamacker/cbor v2.8.0 will require go 1.20 as the minimum version (already done in main branch), so we can skip updating files for go 1.17, etc.

[liggitt]

Started a work-in-progress PR at #644 for discussion, working on unit tests now.

Honoring the omitzero option unconditionally in the cbor tag is straightforward.

I wasn't sure whether to make honoring the omitzero option in the json tag conditional on Go 1.24+ to match stdlib behavior. The Go-version-specific bit could be limited to a single private package boolean, which we can manipulate in tests to simplify maintenance and testing. Can discuss that approach here or in the PR, whichever you prefer.

[fxamacker]

Moved discussion to PR #644.