update: add reset method to Cache interface

Author: FarhanAnjum-opti

core-api/src/main/java/com/optimizely/ab/internal/Cache.java

@@ -21,4 +21,5 @@ public interface Cache<T> {
     int DEFAULT_TIMEOUT_SECONDS = 600;
     void save(String key, T value);
     T lookup(String key);
+    void reset();
 }