Refactor project for easy adding support of new database engine without sqlc core changes

[asmyasnikov]

@kyleconroy What do you think of using an external database engine as a plugin, similar to a database/sql or GORM (for example using "_" import for register engine)?

Originally posted by @asmyasnikov in https://github.com/sqlc-dev/sqlc/issues/4090#issuecomment-3265158841

AI suggest next plan: https://deepwiki.com/search/what-needs-to-be-done-with-the_bc38a550-304d-453f-99a0-071a46160042?mode=fast

Looks doable