Add option to read credentials from ENV

[shhnwz]

Hi Team,

I really appreciate the quality and the usability of this extension. I am using it for building s3 tiering extension and willing to contribute towards enhancing credential provisioning for mapping user. Currently it supports only user and password mode.

New proposed option CREATE SERVER parquet_s3_srv FOREIGN DATA WRAPPER parquet_s3_fdw OPTIONS (use_credential_provider 'true');

Let me know if something like that already exists but I missed it during code-walk through, or it's already in the roadmap in near future.