fix: SA authentication and change how errors in requests are handled

[ondbeh]

There was a change in how the STACKIT SDK handles authentication with service accounts that needed to be reflected in the webhook.
The main change is how we handle errors that occur during the records CRUD operations - instead of trying every remaining request, we cancel them and let external DNS handle the retries.
I have also changed the ordering of which records are created first to avoid orphaned A and CNAME records by creating the TXT ownerships first and deleting them last.