Add trigger binding to offset, timestamp as params

[fbeltrao]

Changes

• Polishes the automatic binding of topic, partition, offset, timestamp and key in trigger functions
• Sample function modified to illustrate how to use automatic binding

Sample:

[FunctionName(nameof(PageViews))]
public static void PageViews(
    [KafkaTrigger("LocalBroker", "pageviews", AvroSchema = PageViewsSchema, ConsumerGroup = "azfunc")] KafkaEventData[] kafkaEvents,
    long[] offsetArray,
    int[] partitionArray,
    string[] topicArray,
    DateTime[] timestampArray)
{
    // offsetArray will contain the offsets of each message
    // same for partitionArray, topicArray and timestampArray
}

Part of #44

[brandonh-msft]

I'm not convinced this adds value to anything if the same information is available off an individual KafkaEventData but I'll trust your judgement ;)

[fbeltrao]

It is useful if you are binding to poco objects. I am mirroring the same feature from EventHubs trigger