feat: add support for modular models metadata

[ewanharris]

Description

This PR introduces the metadata required for modular models. Namely, the file and module fields on TypeDefintions, Relations, and Conditions.

I'm very much a protobuf noob so please call out any extra additions that are needed, I tried to keep the validation aligned with what exists currently.

References

Closes #137

See the updated RFC PR for an example of these additions openfga/rfcs#18

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[jon-whit]

@ewanharris would you please, for the record, include some examples of these protobufs for some file and module structures? That will make future me understand the field structure of these changes better (when I reflect back on this), and it will make it easier to review at the current time as well.