squash - Pluggable Rule Engine: Add Metadata Guard

Author: alanking

docs/plugins/pluggable_rule_engine.md

@@ -11,6 +11,7 @@ The iRODS Consortium currently supports the following rule engine plugins:
  - [C++ Default Policy Rule Engine Plugin](https://github.com/irods/irods/blob/main/plugins/rule_engines/src/cpp_default_policy.cpp)
  - [C++ Audit (AMQP) Rule Engine Plugin](https://github.com/irods/irods_rule_engine_plugin_audit_amqp)
  - [C++ Logical Quotas Rule Engine Plugin](https://github.com/irods/irods_rule_engine_plugin_logical_quotas)
+ - [C++ Metadata Guard Rule Engine Plugin](#metadata-guard-rule-engine-plugin)
  - [C++ Storage Tiering Rule Engine Plugin](https://github.com/irods/irods_capability_storage_tiering)
  - [C++ Indexing Rule Engine Plugin](https://github.com/irods/irods_capability_indexing)