CertConfig is missing autoRegistration for issuers

[MartinWeindel]

How to categorize this issue?

/area control-plane
/kind bug

What happened:
On creating an issuer with exteranl account binding, it was not possible to specify the autoRegistration field.

- type: shoot-cert-service
  providerConfig:
    apiVersion: service.cert.extensions.gardener.cloud/v1alpha1
    kind: CertConfig
    shootIssuers:
      enabled: true
    issuers:
      - email: ***********@***.com
        autoRegistration: true
        externalAccountBinding:
          keyID: *********************
          keySecretName: some-credentials
        name: cdf-dev-sap
        server: https://***.com/***/v1/acme/v2/directory

What you expected to happen:
Should allow to provide autoRegistration: true

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• Gardener version (if relevant):
• Extension version:
• Kubernetes version (use kubectl version):
• Cloud provider or hardware configuration:
• Others: