Aggregations: Added pre and post offset to histogram aggregation

Added preOffset and postOffset parameters to the API for the histogram aggregation which work in the same way as in the date histogram

Closes #6605
> TimeZoneRounding.Builder expects everything thats returned to be an instance of TimeZoneRounding 

TimeZoneRounding doesn't add anything on top of Rounding so I think it's ok to return/expect Rounding instances here.