Add SortMode to encode struct fields in a less predictable order.

[benluddy]

Description

PR Was Proposed and Welcomed in Currently Open Issue

• This PR was proposed and welcomed by maintainer(s) in issue #___
• Closes or Updates Issue feature: Add a new SortMode to encode struct fields in a less predictable order #514

Checklist (for code PR only, ignore for docs PR)

• Include unit tests that cover the new code
• Pass all unit tests
• Pass all lint checks in CI (goimports, gosec, staticcheck, etc.)
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  Last line of each commit message should be in this format:
  Signed-off-by: Firstname Lastname [email protected]
• Certify the Developer's Certificate of Origin 1.1
  (see next section).

Certify the Developer's Certificate of Origin 1.1

• By marking this item as completed, I certify
  the Developer Certificate of Origin 1.1.

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[fxamacker]

@benluddy Nice! 👍 Thanks for adding this option!

I remember this change getting announced by the Go team (~12 years ago) and some didn't understand why it was a good idea at the time.

• https://codereview.appspot.com/5285042/

It is less frequent these days but this issue still resurfaces once in a while (not always related to Go or CBOR) because new developers make assumptions that create dependencies on implementation details (e.g. when unsorted order is not defined by a compiler, codec, etc.).

Thanks again for adding this!