Create uniqueness check for the data

[Nyzl]

I know there's an issue with GA4 data not having a primary key but I think a nice-to-have at some point would be a uniqueness check on a combination of fields that are expected to be unique in the output.

assertions: {
        uniqueKey: ["col_1", "col_2", "col_3"]
    }

Originally posted by @aaronfowles in #40 (comment)