feat: add ca-injector controller for cert-controller-manager-next-generation

[janosbinder]

Summary

• This is my first PR and it was generated using AI (Claude by Anthropic) via the Claude Code CLI.
• Resolves ca-injector feature #185
• Adds a new cainjector package under pkg/certman2/controller/cainjector/ with four per-kind controllers (ValidatingWebhookConfiguration, MutatingWebhookConfiguration, CustomResourceDefinition, APIService) that automatically populate caBundle fields from a CA source driven by annotations
• Adds CAInjectorControllerConfig to CertManagerConfiguration (disabled by default; enable with controllers.caInjector.enabled: true)
• Registers apiregistration/v1 (APIService) in ClusterScheme
• Wires the controllers into cmd/cert-controller-manager-next-generation/app/app.go
• Adds docs/usage/ca-injector.md with full feature documentation
• Adds a ## CA Injector section to README.md

Annotations introduced

Annotation	On resource	Meaning
cert.gardener.cloud/inject-ca-from	Injectable	Inject ca.crt from the Secret backing the named Certificate
cert.gardener.cloud/inject-ca-from-secret	Injectable	Inject ca.crt directly from the named Secret
cert.gardener.cloud/allow-direct-injection	Secret	Must be "true" to permit direct-secret injection

Test plan

• 7 Ginkgo specs covering all four resource kinds, both annotation paths, the allow-direct-injection guard, and requeue-on-missing-CA behaviour
• go test ./pkg/certman2/... — all packages green
• go build ./cmd/cert-controller-manager-next-generation/... — binary builds clean

[gardener-prow]

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[gardener-prow]

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[gardener-prow]

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[marc1404]

/kind task
/kind enhancement
/ok-to-test

[MartinWeindel]

@janosbinder
Thanks for your effort.
Unfortunately, this PR suggests changes to the next generation of the cert-controller-manager, which is currently incomplete and untested. As we have no resources, further development is currently halted until 2027.
This means, even if we would merge this PR in some final shape, it would not be available in the current (legacy) implementation.

[MartinWeindel]

As the legacy controller is based on the legacy controller-manager-library, an implementation of this feature would look quite different. It needs to be integrated into cmd/cert-controller-manager/main.go using configuration and packages from the controller-manager-library.