[wpiutil] DataLog: Add last value and change detection (#6674)

Update() checks/updates the last value and appends only if changed.
GetLastValue() gets the last value.

Also add OutputStream support to Java DataLogWriter.

wpiutil/src/main/native/include/wpi/DataLogWriter.h

@@ -81,6 +81,13 @@ class DataLogWriter final : public DataLog {
    */
   void Stop() final;
 
+  /**
+   * Gets the output stream.
+   *
+   * @return output stream
+   */
+  wpi::raw_ostream& GetStream() { return *m_os; }
+
  private:
   bool BufferFull() final;