add method 'required()'

Author: sergeycherkasovv

app/src/main/java/hexlet/code/schemas/BaseSchema.java

@@ -3,6 +3,7 @@
 
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Objects;
 import java.util.function.Predicate;
 
 public class BaseSchema<T> {
@@ -24,4 +25,10 @@ public final boolean isValid(T value) {
                 .stream()
                 .allMatch(t -> t.test(value));
     }
+
+
+    public BaseSchema required() {
+        addValidation("required", Objects::nonNull);
+        return this;
+    }
 }