Add support for injecting CA from secret for trust manager Webhook

[zeusal]

Add support for injecting CA from Secret, allowing cainjector to run with --enable-certificates-data-source=false as recommended by best practices. https://cert-manager.io/docs/concepts/ca-injector/#controlling-which-data-sources-are-enabled

[cert-manager-prow]

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[cert-manager-prow]

Hi @zeusal. Thanks for your PR.

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[erikgb]

@zeusal, did you also read the note in https://cert-manager.io/docs/installation/best-practice/#memory?

[zeusal]

@zeusal, did you also read the note in https://cert-manager.io/docs/installation/best-practice/#memory?

Yes, I did read the note in the best practices guide. The MR is aligned with it: by allowing inject-ca-from-secret, we can keep running cainjector with --enable-certificates-data-source=false, so the CA is injected from a pre-generated secret instead of generating certificates in the pod. This follows the memory and security recommendations from the guide.

The MR doesn’t change the default behavior — it only adds an optional way to provide the CA from a secret, keeping the pod lightweight and compliant with the best practices.

With this change, the chart can natively support both behaviors. It allows avoiding certificate auto-discovery and additional watchers, while still letting the MutationWebhook obtain the CA from a secret by specifying ca-from-secret and setting cert-manager.io/allow-direct-injection: "true" on that secret.

[erikgb]

I am not a big fan of this "enum", but your use case makes a lot of sense. Are you able to see a way to avoid this? Could it make sense to unconditionally use the cert-manager.io/inject-ca-from-secret annotation on the webhook?