cmd/definitions: Implement GSP-725 Add Defaultable Property for Pair

[JinnyYi]

ref: #727

[codecov]

Codecov Report

Merging #730 (a33768e) into master (46f8633) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #730      +/-   ##
==========================================
- Coverage   12.55%   12.50%   -0.06%     
==========================================
  Files          22       22              
  Lines        1473     1479       +6     
==========================================
  Hits          185      185              
- Misses       1281     1287       +6     
  Partials        7        7              

Flag	Coverage Δ
unittests	12.50% <0.00%> (-0.06%)	⬇️

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Impacted Files	Coverage Δ
tests/generated.go	2.22% <0.00%> (-0.03%)	⬇️

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[Xuanwo]

It looks like WithDefaultStorageClass is not generated.

[JinnyYi]

It has been generated in https://github.com/beyondstorage/go-storage/pull/730/files#diff-d872ad7215a358bcc5ae66b3577ab2db501f7f8497ac6624e7374a19b7f69fedR88. We pointed it as a defaultable pair in namespace before, so we can't find it in this change.