fix: add hashCode implementation

Add a hashCode implementation that ensures that it is in sync with
the equals method.

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/Options.java

@@ -1008,6 +1008,11 @@ void appendToOptions(Options options) {
       options.lastStatement = true;
     }
 
+    @Override
+    public int hashCode() {
+      return LastStatementUpdateOption.class.hashCode();
+    }
+    
     @Override
     public boolean equals(Object o) {
       return o instanceof LastStatementUpdateOption;