Map topic fields to Elasticsearch index fields

[pavelpe]

Add support to map topic fields names to ES field names.
It could be even be a mapping file which is later could be placed in etc/kafka-connect-elasticsearch/config (for example) and then used by ES sink connector.

Currently the only option, as I know, it to force topic value field names to match those in ES index.
But it less convenient because this way if, for instance, I have topic that doesn't match index mapping, then I have to stream all my data through another topic which is adjusted to ES index mapping.

If ES index mapping is changed, then instead of adjusting ES sink connector to new changes and continue consuming, I have to create new stream which puts new data to new topic which is adjusted to latest ES Index changes and update ES Sink connector.