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/implementation/policyenforcer/cmd/plugin.go

@@ -0,0 +1,26 @@
+// Package main provides the plugin entry point for the Policy Enforcer plugin.
+// This file is compiled as a Go plugin (.so) and loaded by beckn-onix at runtime.
+package main
+
+import (
+	"context"
+
+	"github.com/beckn-one/beckn-onix/pkg/plugin/definition"
+	"github.com/beckn-one/beckn-onix/pkg/plugin/implementation/policyenforcer"
+)
+
+// provider implements the PolicyEnforcerProvider interface for plugin loading.
+type provider struct{}
+
+// New creates a new PolicyEnforcer instance.
+func (p provider) New(ctx context.Context, cfg map[string]string) (definition.PolicyEnforcer, func(), error) {
+	enforcer, err := policyenforcer.New(cfg)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return enforcer, enforcer.Close, nil
+}
+
+// Provider is the exported symbol that beckn-onix plugin manager looks up.
+var Provider = provider{}