[kafka] bootstrapServers and Topics to accept arrays

[daisy-ycguo]

Because kafkasource: change bootstrapServers and Topics to accept arrays is enabled in back-end, we shall modify client to support the changes.

Or else, we will get failure when creating a kafka source:

$./kn source kafka create mykafkasrc --servers my-cluster-kafka-bootstrap.kafka.svc:9092 --topics test-topic --consumergroup test-consumer-group
Error: cannot create KafkaSource 'mykafkasrc' in namespace 'default' because: admission webhook "defaulting.webhook.kafka.sources.knative.dev" denied the request: mutation failed: cannot decode incoming new object: json: cannot unmarshal string into Go struct field KafkaSourceSpec.spec.bootstrapServers of type []string

[navidshaikh]

/assign