Refactor Policy Enforcer to Policy Checker

- Renamed the `PolicyEnforcer` interface and related implementations to `PolicyChecker` for clarity and consistency.
- Updated configuration keys in YAML files to reflect the new `checkPolicy` terminology.
- Adjusted related code, tests, and documentation to support the new naming convention and ensure compatibility.
- Enhanced comments and examples for the `checkPolicy` configuration to improve usability.

install/build-plugins.sh

@@ -23,7 +23,7 @@ plugins=(
     "schemav2validator"
     "signer"
     "signvalidator"
-    "policyenforcer"
+    "opapolicychecker"
 )
 
 for plugin in "${plugins[@]}"; do