Mark the key attribute of rotated secrets as computed

[phy1729]

For a rotated secret with the key left as null, a plan after the initial apply will show a diff

      - key                        = "acc-axxxxxxxxxxm__account-def-secrets-key__" -> null

This marks the key attribute as computed in addition to optional to let Terraform know the value may be set by the practitioner, but if left unset the provider may set a value.