cscore: Add Shutdown() function

This allows ordered shutdown / destruction of sources and sinks prior to
global singleton destruction.

cscore/src/main/native/cpp/Instance.h

@@ -51,6 +51,8 @@ class Instance {
 
   static Instance& GetInstance();
 
+  void Shutdown();
+
   wpi::Logger logger;
   Notifier notifier;
   Telemetry telemetry;