difference-of-squares: add starter implementation

exercises/difference-of-squares/src/main/java/DifferenceOfSquaresCalculator.java

+class DifferenceOfSquaresCalculator {
+
+    int computeSquareOfSumTo(int input) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+    int computeSumOfSquaresTo(int input) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+    int computeDifferenceOfSquares(int input) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+}