Allow comments in authorization model DSL

[aaguiarz]

As a developer writing a model, I'd want to add comments to better explain it.

The OpenFGA DSL and the Store API do not support comments.

To address this issue, we need to:

1. Update the API JSON to add a field for comments for relations and types
2. Update the DSL and the parser to translate comments typed and send them to the API and interpreting them back

We should also update the documentation to add descriptive comments for complex models.