PostgreSQL has support for extensions. Installed extensions can be listed in psql with \dx.

Also useful are the system catalogs pg_extension, pg_available_extensions and pg_available_extension_versions.

Extensions can provide their own objects, eg. some functions might be provided by extension, so pgdiff should know that an extension needs to be installed/upgraded and not list those objects (functions/views/etc.) in regular diff.

Easy extension to test this is pg_stat_statements, which has two functions and a view.