Make COPY extendable in order to support Parquet and other formats

Hi hackers,

In several conversations I had recently with colleagues it was pointed
out that it would be great if PostgreSQL supported COPY to/from
Parquet and other formats. I've found a corresponding discussion [1]
on pgsql-general(at)(dot) The consensus reached back in 2018 seems to be that
this shouldn't be implemented in the core but rather an API should be
provided for the extensions. To my knowledge this was never
implemented though.

I would like to invest some time into providing a corresponding patch
for the core and implementing "pg_copy_parquet" extension as a
practical example, and yet another, a bit simpler, extension as an API
usage example for the core codebase. I just wanted to double-check
that this is still a wanted feature and no one on pgsql-hackers@
objects the idea.

Any feedback, suggestions and ideas are most welcome.

[1]: https://postgr.es/m/20180210151304.fonjztsynewldfba%40gmail.com

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Best regards,
Aleksander Alekseev