Update postgres_hook.py

[KarunakarGumudala]

copy_expert works for flat files but not compressed,
copy_expert_gzip can copy the gzipped files to postgres table

Make sure you have checked all steps below.

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[feluelle]

Welcome @KarunakarGumudala !

I like your feature 👍 but I would like to point you to a few steps that you need to change/setup first.

• Please associate a ticket to this change as described in the PR template.
• Also please read the Contributing guide to setup your development environment so that you can test your changes accordingly.

Thank you :)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.