Add schema import for GraphQL SDL

**Is your feature request related to a problem? Please describe.**
As a user of System Designer I’d like to import an existing GraphQL SDL file so I do not have to re-enter all my types / model.

**Describe the solution you'd like**

In my case model of some existing systems already described in an GraphQL SDL file, c.f.: https://graphql.org/learn/schema/ – If you provide a JSON file of the Starwars examples used in the docs, I can provide the model as SDL.

**Describe alternatives you've considered**
Of course one could also write a external converter transforming SDL to MSON, but I think in integrated importer would be more useful.

**Additional context**
TBD