Implement UNLOAD

[jleclanche]

https://docs.aws.amazon.com/redshift/latest/dg/r_UNLOAD.html

Doing so will require patching the postgres source.

Example commits and relevant code:

• Grammar entry point
• kwlist.h
• MERGE implementation commit
• COPY statement
• COPY logic

[dschiavu]

I would love to see UNLOAD implemented here, perhaps using https://github.com/opt-tech/redshift-fake-driver.