Override hashCode in Quantity

### What's done:
- overrode `hashCode` in `Quantity`
- added test which compares two sets of equal elements

It closes kubernetes-client#3490

kubernetes/src/main/java/io/kubernetes/client/custom/Quantity.java

@@ -85,10 +85,14 @@ public boolean equals(Object o) {
     }
 
     Quantity otherQuantity = (Quantity) o;
-
     return ObjectUtils.compare(this.number, otherQuantity.number) == 0;
   }
 
+  @Override
+  public int hashCode() {
+    return Objects.hash(number);
+  }
+
   public static class QuantityAdapter extends TypeAdapter<Quantity> {
     @Override
     public void write(JsonWriter jsonWriter, Quantity quantity) throws IOException {

kubernetes/src/test/java/io/kubernetes/client/custom/QuantityTest.java

@@ -0,0 +1,33 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+http://www.apache.org/licenses/LICENSE-2.0
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+package io.kubernetes.client.custom;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class QuantityTest {
+
+  @Test
+  void containsInSet() {
+    final List<String> values = List.of("0.5Gi", "0.5G", "1Gi", "1G", "500Mi", "500M", "500m", "0.5", "1");
+    final Set<Quantity> set1 = values.stream().map(Quantity::fromString).collect(Collectors.toSet());
+    final Set<Quantity> set2 = values.stream().map(Quantity::fromString).collect(Collectors.toSet());
+
+    assertEquals(set1, set2, "Sets of equal elements are not equal");
+  }
+}