Add @Incubating annotation (#3790)

Fixes #3647

Co-authored-by: Dariusz Jędrzejczyk <dariusz.jedrzejczyk@broadcom.com>

reactor-core/src/main/java/reactor/util/annotation/Incubating.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2024 VMware Inc. or its affiliates, All Rights Reserved.
+ *
+ * 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
+ *
+ *   https://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 reactor.util.annotation;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Signifies that the annotated functionality is currently in a developmental phase.
+ * An API marked as {@code @Incubating} is subject to potential modifications (including removal) without prior notice.
+ *
+ * @author Violeta Georgieva
+ * @since 3.7.0
+ */
+@Target({ElementType.PACKAGE, ElementType.TYPE, ElementType.ANNOTATION_TYPE, ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.FIELD})
+@Retention(RetentionPolicy.RUNTIME)
+@Documented
+public @interface Incubating {
+}