Support referencing tuple file in the store file

[rhamzeh]

Currently, we support a model_file option in addition to model for users who prefer to edit their model in a separate file.

We should support a tuple_file that does the same for tuples.

That file will be the same format as accepted by write

name: Some Store
model_file: ./model.fga
tuple_file: ./tuples.yaml
tests:
  - name: test 1
    tuple_file: ./test1_tuples.json
    check: ...
    list_objects: ...
  - name: test 2
    tuple_file: ./test2_tuples.json
    check: ...
    list_objects: ...

This follows on the work completed on the CLI: openfga/cli#179

On VS Code, this includes:

• Do not mark tuple_file as invalid, both globally and in tests
• Validate tests (Checks and List objects) according to the tuples, including those in the model file

[d-jeffery]

tuple_file fields no longer marked invalid as long as file being referenced exists. As of #147

[mikesouza]

Would definitely be nice to have the second task finished, since I assume it's why we get this warning using the extension:

Reproducible by simply using the FGA CLI testing docs example and moving the tuples to ./tuples.yaml.

[gurleensethi]

Getting the same warning as @mikesouza above.