Support generation of migration metadata

As a developer working within a team I want to be able to track what has changed between migrations.

This could be persisted in a `migrations` directory in the project.

```
├── migrations
│ ├── migration001
│ │ ├── migration001.sql
│ ├── ├── migration001.model.graphql
│ ├── migration002
│ │ ├── migration002.sql
│ ├── ├── migration002.model.graphql
```

This is a key database migration feature currently missing from `graphql-migrate`.

Is there a plan to support this or what is the reason for the decision to not include it?