Add factory reset event

Add `kFactoryReset` event and post it at the beginning of
`ScheduleFactoryReset`.

This event may be useful to application as it provides clean way to
perform additional cleanup steps during factory reset.

Signed-off-by: Adrian Gielniewski <adrian.gielniewski@nordicsemi.no>

Author: adigie

src/app/server/Server.cpp

@@ -616,8 +616,22 @@ void Server::GenerateShutDownEvent()
     PlatformMgr().ScheduleWork([](intptr_t) { PlatformMgr().HandleServerShuttingDown(); });
 }
 
+void Server::PostFactoryResetEvent()
+{
+    DeviceLayer::ChipDeviceEvent event;
+    event.Type = DeviceLayer::DeviceEventType::kFactoryReset;
+
+    CHIP_ERROR error = DeviceLayer::PlatformMgr().PostEvent(&event);
+    if (error != CHIP_NO_ERROR)
+    {
+        ChipLogError(AppServer, "Posting kFactoryReset event failed with %" CHIP_ERROR_FORMAT, error.Format());
+    }
+}
+
 void Server::ScheduleFactoryReset()
 {
+    PostFactoryResetEvent();
+
     PlatformMgr().ScheduleWork([](intptr_t) {
         // Delete all fabrics and emit Leave event.
         GetInstance().GetFabricTable().DeleteAllFabrics();

src/app/server/Server.h

@@ -458,6 +458,8 @@ class Server
     void OnPlatformEvent(const DeviceLayer::ChipDeviceEvent & event);
     void CheckServerReadyEvent();
 
+    void PostFactoryResetEvent();
+
     static void OnPlatformEventWrapper(const DeviceLayer::ChipDeviceEvent * event, intptr_t);
 
 #if CHIP_CONFIG_PERSIST_SUBSCRIPTIONS

src/include/platform/CHIPDeviceEvent.h

@@ -255,6 +255,11 @@ enum PublicEventTypes
      * Signals that secure session is established.
      */
     kSecureSessionEstablished,
+
+    /**
+     * Signals that factory reset has started.
+     */
+    kFactoryReset,
 };
 
 /**