feat: Add tools/list-return-structs

[gmlewis]

This tool directly answers the questions raised here:
#4460 (comment)

This tool shows that there are 615 structs in this repo that are used strictly as return values and are never included as input parameters to opts or body values, and are therefore only unmarshaled in the non-test Go source files.

Out of these 615 return-only structs, 487 of them include at least one field that uses the json "omitempty" or "omitzero" option.

As correctly pointed out by @stevehipwell, the omitempty or omitzero in these 487 structs are unnecessary and can be removed.

Although, as @Tens1des pointed out, the omitempty is nice to have there when using testJSONMarshal so that a null is not provided in the output.

So I'm a bit torn on this issue and am leaning toward keeping the omitempty/omitzero for unmarshal-only structs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.51%. Comparing base (c4176b1) to head (8f453d9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4478   +/-   ##
=======================================
  Coverage   98.51%   98.51%           
=======================================
  Files         195      195           
  Lines       17691    17691           
=======================================
  Hits        17429    17429           
  Misses        262      262           

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