Allow configuring CA certificate lifetime

[sbernauer]

Extracted from #350 (comment)

This might be a breaking change for stackabletech/issues#504

[nightkr]

This shouldn't be breaking, since we can default to the current values.

[sbernauer]

Maybe, but maybe we also want to move all that stuff under a common key, such as management. How knows :D

spec:
  backend:
    autoTls:
      ca:
        secret:
          name: secret-provisioner-tls-ca
          namespace: default
        # Change
        autoGenerate: true
        # to something like
        management:
          enabled: true
          caCertLifetime: 2y
          caCertRenewalThreshold: 1y