feat: Changed to avoid duplicating stuttered field definition

[smeadows-okta]

Added support for contextual tuple to assertions

Description

Currently, assertions are pure (object, relation, subject) tuples. Added support for contextual tuples to assertions for advanced scenarios.

References

946

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]
• [x] 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]

Note: These changes affect https://github.com/openfga/openfga/blob/main/pkg/server/commands/write_assertions.go#L68-L73 (which is already in main of OpenFGA). This code is not yet formally released yet, and we have no public contract on it as of yet, but before we release our next release we'll need to fix that code.