Implement Policy Enforcer Plugin

- Added a new Policy Enforcer plugin to evaluate incoming messages against OPA policies.
- Configurable via YAML with options for policy sources, actions, and query.
- Integrated into existing configuration files for BAP and BPP.
- Updated related tests and documentation for the new functionality.
- Enhanced plugin manager to support Policy Enforcer instantiation.

pkg/plugin/definition/policyEnforcer.go

@@ -0,0 +1,17 @@
+package definition
+
+import (
+	"context"
+
+	"github.com/beckn-one/beckn-onix/pkg/model"
+)
+
+// PolicyEnforcer interface for policy enforcement on incoming messages.
+type PolicyEnforcer interface {
+	Run(ctx *model.StepContext) error
+}
+
+// PolicyEnforcerProvider interface for creating policy enforcers.
+type PolicyEnforcerProvider interface {
+	New(ctx context.Context, config map[string]string) (PolicyEnforcer, func(), error)
+}