fix: mock sync producer does not handle the offset while sending messages

[XSAM]

Mock sync producer does not handle the offset while invoking SendMessages.

e.g.

mockSyncProducer := mocks.NewSyncProducer(t, sarama.NewConfig())

msgs := []*sarama.ProducerMessage{
	{Key: sarama.StringEncoder("foo")},
}
mockSyncProducer.SendMessages(msgs)
// Print offset
print(msgs[0].Offset)

[XSAM]

@bai @d1egoaz Shall we merge it?