Allow Parallel overload for redshift.unload_to_files method

[DylDelport]

Is your feature request related to a problem? Please describe.
Currently, this is hardcoded and produces multiple files in the S3 destination. Users may want the option to only generate one file.

Describe the solution you'd like
Similar to the other overloads, covert this to a parameter.

Describe alternatives you've considered
The alternative is to not use this method and run the UNLOAD statement as an SQL statment.

Additional context

P.S. Please do not attach files as it's considered a security risk. Add code snippets directly in the message body as much as possible.

[kukushking]

Hi @DylDelport thanks for opening this - added in #2507.

Note parallel=False does not guarantee a single file will be produced as they will be split into chunks of max 6.2 GB. More in User guide.

[DylDelport]

Thanks @kukushking