Add config to customize file extension for JsonFormat

[ozgenbaris1]

Hi,

It would be a nice option if we could simply modify filename extension for JsonFormat as in ByteArrayFormat. When I search the docs, I can see there is a config named format.bytearray.extension. The same approach could be made for JsonFormat as well. (format.json.extension maybe) Because the files are stored in ndjson format in s3, it would be better for me to store them with .ndjson extension for consistency.