gzip csv files don't work with our custom sql generation

there are a few options here:
- error out before the database tries to read anything if there isn't a way to do this natively
- error out on everything except postgres + UNIX OSs (postgres has a `PROGRAM` clause in its `COPY` command that allows use of an external program to e.g., gzcat the file in)
- do the above for postgres, and for everything else enforce this path `csv -> chunks(temp(csv)) -> sa.Table`
- to enforce this without adding a bunch of conditionals, it might make sense to have a `GZip` metatype

cc @llllllllll @ltransom