Retrieve information from the Superset database directly

[Vitor-Avila]

Currently, the CLI uses APIs or even consume HTML pages to retrieve information from Superset (local), to perform a migration. Even though this approach works, it has some limitations (for example #189), and depending on the operation, it can take a long time to execute (for example, to retrieve the assets' uuids it has to export them).

A valid workaround here would be implementing the ability for the CLI to retrieve this information from the Superset DB, instead. User should be able to specify if the information should be retrieved from the application, or from the DB directly (a connection to the DB would be required).