Deploy additional CAs with the autoTls backend

[siegfriedweber]

Description

It should be possible to deploy additional CAs with the autoTls backend.

If a company wants to use its own root certificate instead of the auto-generated one, an intermediate certificate can be given to the secret operator. Currently, the secret operator only provides this intermediate certificate in volume mounts. Tools, like kcat, which do not use this certificate as a trust anchor, additionally need the root certificate. Therefore, the SecretClass CRD should be extended to allow defining additional certificates, e.g.:

---
apiVersion: secrets.stackable.tech/v1alpha1
kind: SecretClass
metadata:
  name: tls-intermediate-cert
spec:
  backend:
    autoTls:
      ca:
        autoGenerate: false
        secret:
          name: intermediate-ca
          namespace: stackable-operators
      additionalTrustRoots:
        - secret:
            name: root-ca
            namespace: stackable-operators