Add SSE style subscription support to eventservice

This commit adds the SSE style eventservice subscription style event
Using this, end user can subscribe for Redfish event logs using GET
on SSE uris from browser.

Tested:
 - From Browser did GET on above SSE URI and
   generated some Redfish event logs(power cycle)
   and saw redfish event logs streaming on browser.
 - After SSE registration, Check Subscription collections
   and GET on individual subscription and saw desired
   response.
 - Ran RedfishValidation and its passed.

Change-Id: I7f4b7a34974080739c4ba968ed570489af0474de
Signed-off-by: AppaRao Puli <apparao.puli@linux.intel.com>
Signed-off-by: P Dheeraj Srujan Kumar <p.dheeraj.srujan.kumar@intel.com>
Signed-off-by: Ed Tanous <edtanous@google.com>
diff --git a/redfish-core/lib/event_service.hpp b/redfish-core/lib/event_service.hpp
index 5a66c97..02bb21f 100644
--- a/redfish-core/lib/event_service.hpp
+++ b/redfish-core/lib/event_service.hpp
@@ -43,8 +43,6 @@
     "Task"};
 #endif
 
-static constexpr const uint8_t maxNoOfSubscriptions = 20;
-
 inline void requestRoutesEventService(App& app)
 {
     BMCWEB_ROUTE(app, "/redfish/v1/EventService/")
@@ -62,6 +60,9 @@
             "#EventService.v1_5_0.EventService";
         asyncResp->res.jsonValue["Id"] = "EventService";
         asyncResp->res.jsonValue["Name"] = "Event Service";
+        asyncResp->res.jsonValue["ServerSentEventUri"] =
+            "/redfish/v1/EventService/SSE";
+
         asyncResp->res.jsonValue["Subscriptions"]["@odata.id"] =
             "/redfish/v1/EventService/Subscriptions";
         asyncResp->res