chore: contextual tuples use WriteRequestTupleKey

[rhamzeh]

Description

Considering that Contextual Tuples act like tuples that have been written, it makes sense that they use WriteRequestTupleKey.

Opening up the PR for conversations and thoughts

References

Review Checklist

• [x] 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]
• [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]

Contextual tuples just augment the tuples in the database. They only differ in that they are provided at request-time. When we handle a contextual tuple key we pass it straight through and yield it as though it was any other TupleKey from the database. Consequently, if we change this to a WriteRequestTupleKey then we'll have to translate each and every contextual tuple to/from a TupleKey in critical code paths dealing with the TupleKey logic (e.g. core Check and ListObjects paths). For that reason, it makes sense to me to have them represented as a TupleKey (e.g. the same thing the storage layer is returning). This avoids conversions in critical code paths and overloading the struct in various places it is used.