Implement Open Policy Agent (OPA) for RBAC/ABAC

[nathandf]

OPA - https://www.openpolicyagent.org/
OPA WASM crate - https://docs.rs/opa/latest/opa/index.html
rego - https://www.openpolicyagent.org/docs/policy-language

• Define access control policies in rego
• Compile to WASM via Opa CLI (should live in src/devtools)
• Use OPA WASM crate to instantiate and OPA runtime
• application-layer Policy Engine abstraction
• infrastructure-layer OPA Policy Engine abstraction
• bootstrap-layer Policy Engine Pool which provides individual, thread-safe OPA Policy Engine instances (Arc<Mutex>)