Allow to define an existing identity

Hi,

It would be nice to be able to specify a pre-existing Managed Identity to use for DNS control. This is a requirement in my ecosystem as another team manages DNS and if we need to re-create the cluster, the MI will be recreated and would need to be re-granted access to DNS.

Thank you!

Start by tracing how DNS control currently obtains and configures its Managed Identity, then review the operator's cluster recreation path. The change is done when a pre-existing Managed Identity can be specified for DNS control and remains usable without requiring a newly recreated identity to receive access again.

Written by the indexing model from the issue text.