feat: add batch-check proto definition

[justincoh]

Description

This PR adds the proto definition for the forthcoming batch-check endpoint.

The associated swagger documentation update:

References

Overarching batch check RFC is coming soon and will be linked here when it's available.

Review Checklist

• [ ] I have clicked on "allow edits by maintainers".
• [x] 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