feat: Simple OPA support (#644)

* opa-feature

* Update crates/policy/src/policy/opa.rs

* add the content related to 'Copyright'

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Co-authored-by: root <root@debian.localdomain>
Co-authored-by: houseme <housemecn@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

crates/config/src/lib.rs

@@ -32,3 +32,5 @@ pub mod audit;
 pub mod notify;
 #[cfg(feature = "observability")]
 pub mod observability;
+#[cfg(feature = "opa")]
+pub mod opa;