feat: adding conflict error to error definition

[adriantam]

Description

Adding new HTTP return code (409) which will be returned for conflict

References

First step in addressing https://github.com/openfga/openfga/issues/1080 Corresponding change https://github.com/openfga/openfga/pull/1179

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

[adriantam]

Updated to avoid introducing new internal code