Create function that fetches resources that are available to import from `ttd-content`

In order to make the user available to choose which resources to import into the app from the organisation library, we need a function that queries these resources.

## Acceptance criteria
- The backend has a function that returns a list of resources that are available to import into the given app

## Out of scope
- Support for other organisations than `ttd`
- Endpoint that calls this function