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.

policies/compliance.rego

@@ -0,0 +1,18 @@
+package policy
+
+import rego.v1
+
+# Example policy: validate confirm action messages.
+# This is a sample policy file. Replace with your actual business rules.
+#
+# The policy evaluates incoming beckn messages and produces a set of
+# violation strings. If any violations exist, the adapter will NACK
+# the request.
+#
+# Available inputs:
+#   - input: the full JSON message body
+#   - data.config: runtime config from the adapter config (e.g., minDeliveryLeadHours)
+
+# violations is the set of policy violation messages.
+# An empty set means the message is compliant.
+violations := set()