The article mentions PLV8 which transparently converts everything to JSON internally and can be used to output json directly. And there are various other extensions to Postgres that will allow you move JSON into and out of the database.

If you want to talk to an arbitrary webservice as though it were a local table you can use www_fdw (Foreign Data Wrapper) https://github.com/cyga/www_fdw/wiki/Examples

And if you're doing geo stufff PostGIS speaks GeoJSON.