[GEN] sea-orm uses migration based approach

[JonasCir]

General

seo-orm in general uses a migration based approach, which means that migrations provided by the developer are used to generate the entity code.

Since we are completely based on sea-orm we should follow this approach.

Proposed change

Introduce the following new gen commands:

1. gen migration: Use sea-orm-cli migrate generate my_new_table -u programmatically
2. gen model: Use sea-orm-cli migrate fresh programatically (we probably want a dedicated DB for generation and one DB for the user to tests its app)
3. gen graphql: Use templating of GraphQL, later switch to Seaography