Add support for v5 of the pgx Go driver

[kyleconroy]

What do you want to change?

This proposal is a work in progress

v5.0.0 of jackc/pgx is coming in September (https://github.com/jackc/pgx/issues/1273). The changes are significant. I think it's a good opportunity to make some backwards-incompatible changes to fix some long standing issues with our pgx support

What issues should be fixed?

• [ ] https://github.com/kyleconroy/sqlc/issues/1494
• [ ] https://github.com/kyleconroy/sqlc/issues/1507
• [ ] https://github.com/kyleconroy/sqlc/issues/1752
• [ ] https://github.com/kyleconroy/sqlc/pull/1641
• [ ] https://github.com/kyleconroy/sqlc/pull/1651
• [ ] https://github.com/kyleconroy/sqlc/pull/1623

What database engines need to be changed?

PostgreSQL

What programming language backends need to be changed?

Go