Mode config option is not marked as required

[Jeaksen]

When fetching config options from Kafka Connect REST API endpoint
GET /connector-plugins/io.confluent.connect.jdbc.JdbcSourceConnector/config the only entries marked as required are:

• connection.url
• name
• connector.class

However validating a config consisting only of those fields results in an error:

{
    "error_code": 500,
    "message": "org.apache.kafka.common.config.ConfigException: Query mode must be specified"
}

Specifying "mode" option fixes this error, however, it is not marked as required.