Transform optional enums to match pre kube 2.0.0 format

[Danil-Grigorev]

Motivation

Schemars bump to 1.0 with kube 2.0 broke optional enum CRD generation. Verified it with schemars 1.1.0 - problem still exists.

Solution

Introduce an OptionalEnum transform to allow usage of Option<Enum> as in kube 1.0.0 with schemars < 1.0.0.

Removes incompatible output of schema generator with:

{
  "anyOf": [
    { "enum": ["Debug", "Info", "Error"], "type": "string" },
    { "enum": [null], "nullable": true }
  ]
}

replacing it with expected:

{ "enum": ["Debug", "Info", "Error"], "type": "string", "nullable": true }

Fixes: #1821

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.8%. Comparing base (a12559d) to head (e4e239b).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1853     +/-   ##
=======================================
+ Coverage   74.7%   74.8%   +0.1%     
=======================================
  Files         84      84             
  Lines       7938    7951     +13     
=======================================
+ Hits        5927    5941     +14     
+ Misses      2011    2010      -1     

Files with missing lines	Coverage Δ
kube-core/src/schema.rs	95.8% <100.0%> (+0.7%)	⬆️
kube-derive/src/custom_resource.rs	84.3% <ø> (ø)
kube-derive/tests/crd_schema_test.rs	100.0% <ø> (ø)

... and 3 files with indirect coverage changes

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[clux]

this looks sensible to me. minor nits only.
need to look at the bigger PR though.

[clux]

Thanks a lot. Easy to review. Maybe let's wait for a response on #1839 first. They have tests for this exact transform, and it'd be good to preserve those / get some tests for this.

let's wait with this. the #1893 pr is more holistic approach, so lets see if we can get that merged first.