PostgreSQL foreign tables: handle them like views (fixes #30990) [nee…

[alkra]

Description

Fixes #30990

PostgreSQL data provider does not handle the case of a foreign table.

1. Add a case matching for Relkind::ForeignTable
2. Enable fetching the columns, so that the user can choose

Checklist

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[alkra]

Checklist:

• I am unsure whether this is a new feature.
• The commit title seems too long, [needs-docs] is truncated, does it matter?
• I don't know how to write a unit test for this.

(Under WSL; I had to comment out the changes introduced by 9b04a29 for scripts/prepare-commit.sh to run)

[nyalldawson]

I am unsure whether this is a new feature.

No - it's a fix.

The commit title seems too long, [needs-docs] is truncated, does it matter?

Nope, no issue

I don't know how to write a unit test for this.

I don't think it's a requirement for this.