make it easier to run pggen

[ethanpailes]

It is annoying to configure a postgres server to use as part of your build pipeline. pggen should have an option to do that automatically for you.

[ethanpailes]

Originally I was thinking of spawing a docker container, but https://pkg.go.dev/github.com/fergusstrange/embedded-postgres looks like an even easier way to do it. It's a pretty heavy dependency though, and folks still need to tell us how to run migrations somehow. I'm thinking I might want to provide a separate binary so that not everyone has to pull in the full dependency if they don't want to.