Use `admin.` configuration prefix in Kafka Connect configuration

Kafka Connect is currently configured in following way:

• Without prefix for AdminClient used by KafkaConnect it self
• Producer prefix
• Consumer prefix
  However, there seems to be the admin. prefix missing whihc is used by some topics created by connectors. For example the dead letter queue topic. The issue can be reproduced for example by deploying the following connector:

curl -X POST -H "Content-Type: application/json" --data '{ "name": "sink-test", "config": { "connector.class": "FileStreamSink", "tasks.max": "3", "topics": "kafka-test-apps", "file": "/tmp/test.sink.txt", "errors.deadletterqueue.topic.replication.factor": 1, "errors.deadletterqueue.topic.name": "dlq-es-sink",
"errors.deadletterqueue.context.headers.enable": true } }' http://localhost:8083/connectors

Which will crash the connect and in the log you can see that it is trying to authenticate without the correct authentication. This PR should fix it by adding the admin. prefix to the configuration file and using it with the regular configuration for authentication.