[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/core.h b/agent/common/core.h
index b3a5408..09ff890 100644
--- a/agent/common/core.h
+++ b/agent/common/core.h
@@ -243,6 +243,7 @@
 Status GetLogicalOnuDistance_(uint32_t intf_id, uint32_t onu_id, openolt::OnuLogicalDistance* response);
 Status GetOnuStatistics_(uint32_t intf_id, uint32_t onu_id, openolt::OnuStatistics *onu_stats);
 Status GetGemPortStatistics_(uint32_t intf_id, uint32_t gemport_id, openolt::GemPortStatistics* gemport_stats);
+Status GetPonRxPower_(uint32_t intf_id, uint32_t onu_id, openolt::PonRxPowerData* response);
 int get_status_bcm_cli_quit(void);
 uint16_t get_dev_id(void);
 Status pushOltOperInd(uint32_t intf_id, const char *type, const char *state);