Add support for github.com/jackc/pgx

[prochac]

There are two main approaches how to use Postgres in Go. Using the stdlib abstraction *database/sql.DB or *pgx.Conn from github.com/jackc/pgx module.

The challenge is how to support v4, v5 and other future releases of the module. Meanwhile, we don't want to pollute our go.mod with all possible versions.