Improve environment variable support for the pg backend

[remilapeyre]

This patch does two things:

• it adds environment variable support to the parameters that did not have it (and uses PG_CONN_STR instead of PGDATABASE which is actually more appropriate to match the behavior of other PostgreSQL utilities)
• better documents how to give the connection parameters as environment variables for the ones that were already supported based on the recommendation of @bsouth00

I will prepare a backport of the documentation part of this once it is merged.

Closes #33024

Target Release

1.5.0

Draft CHANGELOG entry

The pg backend now supports the PG_CONN_STR, PG_SCHEMA_NAME, PG_SKIP_SCHEMA_CREATION, PG_SKIP_TABLE_CREATION and PG_SKIP_INDEX_CREATION environment variables.

Documentation page

[crw]

Thanks @remilapeyre, I'll send this through traige.

[liamcervante]

Hi @remilapeyre, this mostly looks fine to me. Just a single comment on the databaseName variable. I'm happy to merge once that's resolved!

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[liamcervante]

Thanks @remilapeyre! This is all ready to go as part of v1.5!

[github-actions]

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