ETL: Create a converter api and provide an implementation for Json Object to flat map conversion

The input operators receive data in specific format. When these input operators are plugged in a DAG, the input data format needs to be converted to another. Currently we achieve this by adding downstream operator which just does this transformation. However an operator just for conversion of data format transformation is un-necessary because it wastes resources.

This problem could be addressed by providing a Converter api and couple of common implementations of this Converter which can be set as a property on the operators.