Delete token from Unleash when ApiToken is deleted

[Starefossen]

Delete the API client from Unleash the Unleash instance when the ApiToken resource is deleted
from Kubernetes.

• Extend pkg/unleash/admin_api.go with DeleteApiToken() function
  • https://docs.getunleash.io/reference/api/unleash/delete-api-token
• Implement doFinalizerOperationsForToken() in controllers/apitoken_controller.go