✨feat: add non-fatal warning system for code generation diagnostics

[mattsu2020]

Summary

This PR implements the warning system requested in #434 by adding a non-fatal warning infrastructure that mirrors the existing error reporting system.

• Add Package.Warnings field and AddWarning() method to pkg/loader, following the same error type dispatch pattern as AddError()
• Add PrintWarnings() and VisitPackages() helper functions to pkg/loader
• Add Runtime.PrintWarnings() method to pkg/genall and call it after successful generation in controller-gen
• Emit a warning when pointers are used as map values in CRD schemas (map[string]*string), which is supported but not recommended per Kubernetes API conventions
• Add tests verifying that pointer map values produce a warning and non-pointer map values do not

Test plan

• go build ./... passes
• go test ./pkg/loader/... — 11/11 passed
• go test ./pkg/crd/... — all passed (including new Test_Schema_PointerMapValue_Warning and Test_Schema_NonPointerMapValue_NoWarning)
• Manual test: run controller-gen against a type with map[string]*string and verify warning output on stderr

Closes: #434

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[sbueringer]

I wonder if producing findings like this should be rather implemented in kube-api-linter

@JoelSpeed WDYT?

[mattsu2020]

Thanks

I looked into kube-api-linter and I agree this check fits better there than in controller-tools. There is already a nomaps linter that inspects map types and enforces Kubernetes API design guidance around maps, so I think the closest implementation path would be to extend that linter with an option to report map types whose value type is a pointer, e.g. map[string]*string.

I can move the actual API-convention check to kube-api-linter, likely as a nomaps configuration such as disallowPointerValues, and keep this controller-tools PR closed unless maintainers still want a generic non-fatal warning mechanism here.