Make validator and defaulter more extensible

[matthchr]

As the code generator gets fully fleshed out, we're running into cases where we need to perform special validation or defaulting logic on all resources. Additionally, the ability to manually add to validations/defaults for certain resources will be needed.

See https://github.com/Azure/azure-service-operator/blob/master/docs/design/defaulter-validator.md for the design of this feature.

Closes #1430

[matthchr]

I know this looks like a pretty large PR, but ~2300 lines are golden files and ~200 are just moving code to a new file, so there's really only 600 lines of new code here.

[codecov-commenter]

Codecov Report

Merging #1507 (7eb179d) into master (0423ffd) will increase coverage by 0.92%.
The diff coverage is 91.01%.

❗ Current head 7eb179d differs from pull request most recent head 6fc2b85. Consider uploading reports for the commit 6fc2b85 to get more accurate results

@@            Coverage Diff             @@
##           master    #1507      +/-   ##
==========================================
+ Coverage   62.81%   63.73%   +0.92%     
==========================================
  Files         165      169       +4     
  Lines       10925    11265     +340     
==========================================
+ Hits         6862     7180     +318     
- Misses       3419     3436      +17     
- Partials      644      649       +5     

Impacted Files	Coverage Δ
hack/generator/pkg/astbuilder/calls.go	89.47% <ø> (+10.52%)	⬆️
...kg/astmodel/object_type_serialization_test_case.go	2.08% <0.00%> (ø)
...rator/pkg/codegen/pipeline_create_storage_types.go	3.15% <0.00%> (ø)
...enerator/pkg/codegen/resource_registration_file.go	0.00% <0.00%> (ø)
hack/generator/pkg/astmodel/function.go	37.50% <37.50%> (ø)
...or/pkg/astmodel/kubernetes_admissions_validator.go	92.40% <92.40%> (ø)
...or/pkg/astmodel/kubernetes_admissions_defaulter.go	95.60% <95.60%> (ø)
hack/generator/pkg/astbuilder/assignments.go	100.00% <100.00%> (ø)
hack/generator/pkg/astbuilder/builder.go	84.71% <100.00%> (+0.09%)	⬆️
hack/generator/pkg/astmodel/arm_spec_interface.go	91.39% <100.00%> (+0.28%)	⬆️
... and 8 more

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[Porges]

Closes #1430?

[matthchr]

So it does! Added to the description