Implement parameter substitution for migrations

[krlohnes]

Does your PR solve an issue?

fixes #3178

Is this a breaking change?

No. The addition of fields to the Migrator is semver exempt, The MigrateError enum is #[non-exhaustive]. The new macro functionality retains the existing functionality of the crate and should be backward compatible. I'm making the assumption that changes to the function signatures of sqlx-cli are not considered breaking, but please correct me if I'm wrong there.