Added SubscribeForMetadata() to the sarama client, to access the header field of each received message.

Also replaced TestKafkaClientImplementsKafkaClientIf() with static checks.

For VOL-2207.  Please consider these related patchsets together:
https://gerrit.opencord.org/#/q/VOL-2207

Change-Id: I77f03fbb0ebcdf60cb5f8d0c8c3276325510e874
diff --git a/pkg/kafka/client.go b/pkg/kafka/client.go
index 6289043..9abad93 100755
--- a/pkg/kafka/client.go
+++ b/pkg/kafka/client.go
@@ -66,6 +66,7 @@
 	DeleteTopic(topic *Topic) error
 	Subscribe(topic *Topic, kvArgs ...*KVArg) (<-chan *ca.InterContainerMessage, error)
 	UnSubscribe(topic *Topic, ch <-chan *ca.InterContainerMessage) error
+	SubscribeForMetadata(func(fromTopic string, timestamp int64))
 	Send(msg interface{}, topic *Topic, keys ...string) error
 	SendLiveness() error
 	EnableLivenessChannel(enable bool) chan bool