[VOL-4069] Measure and read Rx optical power for an ONU

Reading Rx optical power requires triggering an RSSI measurement and tracking the RSSI Measurement Completed indication.
The raw value is converted to mW, and a dBm value is returned back.
A small test application (~40K) is bundled within the Debian package to help with debugging the optical power levels.

Change-Id: I20f304a9de0c47b94dfd7b1d8fdd52c56d6a2983
diff --git a/agent/common/server.cc b/agent/common/server.cc
index 68cce10..168df68 100644
--- a/agent/common/server.cc
+++ b/agent/common/server.cc
@@ -347,6 +347,16 @@
             request->gemport_id(),
             response);
     }
+
+    Status GetPonRxPower(
+            ServerContext* context,
+            const openolt::Onu* request,
+            openolt::PonRxPowerData* response) override {
+        return GetPonRxPower_(
+            request->intf_id(),
+            request->onu_id(),
+            response);
+    }
 };
 
 bool RunServer(int argc, char** argv) {