Overrides for unsigned types

[Jille]

What do you want to change?

Neither database/sql or https://github.com/go-sql-driver/mysql supports nullable unsigned integers.

It is easy to define a custom type for them however, so I'd like to configure overrides to use my custom type.

Currently sqlc generates a sqlc.NullInt32 and ignores the unsignedness.

This probably relates to #3098

What database engines need to be changed?

MySQL

What programming language backends need to be changed?

Go