Inconsistent last modified date format in wrangler connections

We don't seem to be having consistent date format for "last modified date" for files in different connections. Some appear in seconds(gcs) and others in milliseconds and others in string with formatted dates.

This is hard for UI to have a generic functionality to show dates as each browser needs specific logic to show dates in a consistent format. We should return the last modified dates in dataprep connections in milliseconds all the time so that we can format the date we show in ways we want.