feat: Change DatasetArtifact.type from enum to String

[Bento007]

Reason for Change

• Changing DatasetArtifact.type from enum to String will make it easier to add new artifact types in the future without doing a database migration. This is safe because only the backend code has access to the database and will handle enforment.

Changes

• add database migration to change DatasetArtifact.type from an enum to a string
• update orm.py to make DatasetArtifact.type

Testing steps

• tested the migration locally
• Ran the migration on dev and verified existing datasets could be retrieved and that new datasets can be created.

[github-actions]

Deployment Summary

• backend_url: https://pr-7479-backend.rdev.single-cell.czi.technology
• backend_de_url: https://pr-7479-backend-de.rdev.single-cell.czi.technology
• backend_wmg_url: https://pr-7479-backend-wmg.rdev.single-cell.czi.technology
• frontend_url: https://pr-7479-frontend.rdev.single-cell.czi.technology
• delete_db_task_definition_arn: arn:aws:ecs:us-west-2:***:task-definition/dp-rdev-pr-7479-deletion:1
• migrate_db_task_definition_arn: arn:aws:ecs:us-west-2:***:task-definition/dp-rdev-pr-7479-migration:1

[joyceyan]

we can do this in a follow up PR, but it might make sense to use string literals that way you still have some type enforcement but passing strings makes it more flexible for db / API layer.

https://mypy.readthedocs.io/en/stable/literal_types.html