Add some way to specify there is a header for upload connection

We are removing the header option for the parse-as-csv directive because it has a bunch of problems. However, this causes a problem with the upload connection, since there will be no way to specify that the first row is a header.

For this, we still need some way to specify the header, but it should not be done as a directive since that will cause issues everywhere else. For example, it could be specified prior to sampling, similar to file connections.