Add NotMultiProjectCapable annotation (elastic#129934)

Some features are unavailable in serverless and are thus not worth the
investment to make fully project-aware. This new annotation can be used
to clearly mark blocks of code that are intentionally not made properly
project-aware, in case we need to revisit them in the future.

Author: nielsbauman

libs/core/src/main/java/org/elasticsearch/core/NotMultiProjectCapable.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.core;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Annotation to identify a block of code (a whole class, a method, a field, or a local variable) that is intentionally not fully
+ * project-aware because it's not intended to be used in a serverless environment. Some features are unavailable in serverless and are
+ * thus not worth the investment to make fully project-aware. This annotation makes it easier to identify blocks of code that require
+ * attention in case those features are revisited from a multi-project POV.
+ */
+@Retention(RetentionPolicy.SOURCE)
+@Target(
+    { ElementType.LOCAL_VARIABLE, ElementType.CONSTRUCTOR, ElementType.FIELD, ElementType.METHOD, ElementType.TYPE, ElementType.MODULE }
+)
+public @interface NotMultiProjectCapable {
+
+    /**
+     * Some explanation on why the block of code would not work in a multi-project context and/or what would need to be done to make it
+     * properly project-aware.
+     */
+    String description() default "";
+}