Add extension_is_visible_hook (PG 18)

[dcw-data]

Summary

Add extension_is_visible_hook to PostgreSQL that allows extensions to filter which extensions
are visible in pg_available_extensions and pg_available_extension_versions catalog views.

Changes

• src/include/fmgr.h — Add extension_is_visible_hook_type typedef and extension_is_visible_hook extern
• src/backend/utils/fmgr/fmgr.c — Initialize extension_is_visible_hook to NULL
• src/backend/commands/extension.c — Call the hook in pg_available_extensions() and pg_available_extension_versions() to skip extensions where the hook returns false

The hook follows the standard PostgreSQL hook pattern: when NULL (default), behavior is unchanged.
Extensions can set the hook to filter which extensions are visible in the catalog views.