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 #4090 (comment)

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

Looks doable