Always provide session variables to queries

Currently, Hasura will provide session variables to mutations and "custom functions" created with 'track_function V2'.

I would really like Hasura to *always* pass session variables to postgres, whether running queries, mutations or custom functions.

My database is accessed by multiple clients, one of which is Hasura. My other clients use Postgres "configuration settings" (via `set_config`, `current_setting`) to provide authorization information used during queries. I'd like to leverage the same infrastructure when executing queries generated by Hasura.

A similar request was made in #3872.