feat: add limited support for mixed operators in Golang binding

[jimmyjames]

This PR adds initial support for mixed operators to the Golang bindings.

Description

Golang binding implementation for the initial mixed operators support as done in #107. Similar to the JS implementation, this change does not support anything involving multiple references to direct assignment, or the presence of direct assignment not as the first entry, or references to self/this.

References

• Golang implementation of JS bindings in #107
• Closes #132

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

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: jimmyjames / name: Jim Anderson (f2803abce378e6315f5a34bf095c3bbb896cb25a, 5eb8196418c51d1b5d23bac0ba0d11710a0f78b1, a64169bd59d9a8b54c9315603c90f285fdf2661c)