docs: PolicyMonitor DR (eclipse-edc#3441)

* docs: PolicyMonitor DR

* PR remarks

docs/developer/decision-records/2023-09-07-policy-monitor/README.md

@@ -0,0 +1,28 @@
+# Generic Properties
+
+## Decision
+
+A `PolicyMonitor` extension component will be implemented.
+
+## Rationale
+
+Some transfer types, such as streaming never reach the `COMPLETED` state, so they need to be terminated whenever the
+`Policy` on which the participants agreed is not valid anymore.
+
+## Approach
+
+This component will be implemented using the same model as the state machine we're using for `TransferProcess`:
+
+- The ingestion will be done with a listener on the `TransferProcessStarted` event, that will add an entry to a `PolicyMonitorStore`.
+- The state machine then will continuously fetch the oldest entries leasing them, check the related policy. 
+  - If the check fails the TP will get completed through a service call (that could be either embedded in the control-plane 
+    or remote through a rest call) and the entry will be removed from the `PolicyMonitorStore`.
+  - otherwise the lease will be broken and the policy be verified again in the next run.
+
+With "leasing" we mean that the entity will be reserved by a connector instance that will be the only one allowed to modify it
+until the lease gets broken, and this can happen by updating it or after a certain amount of time.
+
+This mechanism will permit to scale the process to multiple instances.
+
+The `PolicyMonitor` will be deployable embedded in the control-plane or separately in a standalone runtime.
+The `PolicyMonitorStore` will have a in-memory implementation for testing scenarios and a sql (postgres) one.

docs/developer/decision-records/README.md

@@ -47,3 +47,4 @@
 - [2023-06-30 State Machine guards](2023-07-20-state-machine-guards)
 - [2023-08-01 Default datasource](2023-08-01-default-datasource)
 - [2023-08-07 Generic properties](./2023-08-07-generic-properties)
+- [2023-09-07 Policy Monitor](./2023-09-07-policy-monitor)