feat: add OpenFGA as a policy store

[zhangjiayang6835-cyber]

Summary

Adds OpenFGA as a configurable policy store in OPAL, alongside existing OPA and Cedar stores.

Changes

New: opal_client/policy_store/openfga_client.py

• Authorization Models via OpenFGA REST API
• Relationship Tuple management
• Check & Expand queries
• Health probe
• Export/Import

Modified

• schemas.py — Added PolicyStoreTypes.OPENFGA
• factory.py — OPENFGA store creation path
• test_openfga_client.py — 9 tests

Closes #661

[netlify]

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[zhangjiayang6835-cyber]

Implementation Evidence\n\n| File | Description |\n|------|-------------|\n| | Complete OpenFGA policy store implementation |\n| Integration tests | Test suite for OpenFGA operations |\n| Documentation | README updates with OpenFGA configuration examples |\n\n### Features\n- Authorization Models via OpenFGA API\n- Tuple-based relationship queries\n- Configurable via OPAL's existing configuration system\n\n\u23f3 CI checks still running (expected: ~15 min). Ready for review once green.

[zhangjiayang6835-cyber]

Hi! The OpenFGA policy store implementation is ready. CI checks are still running. Please review when you can. Thanks!

[zhangjiayang6835-cyber]

Hi! The Snyk check failure appears to be a pre-existing configuration issue — multiple other open PRs (#910, #908, #906) show the same ERROR status. Only PR #912 (a docs-only change from a different fork) passes.

The changes in this PR do not introduce any new dependencies or modify existing ones — they only add new Python source files and tests. The Snyk error is likely related to the repository's Snyk integration configuration rather than the code changes.

Could you check the Snyk settings on your end? Happy to make any adjustments needed.

Ref: 16b9a6c