[api] Define standard property for handling duplicated keys

Signed-off-by: Jorge Bescos Gascon <jorge.bescos.gascon@oracle.com>

api/src/main/java/jakarta/json/JsonConfig.java

diff --git a/api/src/main/java/jakarta/json/JsonConfig.java b/api/src/main/java/jakarta/json/JsonConfig.java
new file mode 100644
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+++ b/api/src/main/java/jakarta/json/JsonConfig.java
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+/*
+ * Copyright (c) 2021 Oracle and/or its affiliates. All rights reserved.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0, which is available at
+ * http://www.eclipse.org/legal/epl-2.0.
+ *
+ * This Source Code may also be made available under the following Secondary
+ * Licenses when the conditions for such availability set forth in the
+ * Eclipse Public License v. 2.0 are satisfied: GNU General Public License,
+ * version 2 with the GNU Classpath Exception, which is available at
+ * https://www.gnu.org/software/classpath/license.html.
+ *
+ * SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
+ */
+
+package jakarta.json;
+
+/**
+ * This class contains the Json properties and values.
+ *
+ * @since 2.1
+ */
+public final class JsonConfig {
+
+    /**
+     * Configuration property to define the strategy for handling duplicate keys.
+     *
+     * See {@link KeyStrategy}
+     */
+    public static final String KEY_STRATEGY = "jakarta.json.JsonConfig.keyStrategy" ;
+
+    /**
+     * It avoids new instances of this class.
+     */
+    private JsonConfig() {}
+    
+    /**
+     * Contains the different values allowed for {@link #KEY_STRATEGY}.
+     *
+     * See {@link #KEY_STRATEGY}
+     */
+    public static enum KeyStrategy {
+        /**
+         * Configuration value that will take the value of the first match.
+         */
+        FIRST,
+        /**
+         * Configuration value that will take the value of the last match.
+         */
+        LAST,
+        /**
+         * Configuration value that will throw {@link JsonException} when duplicate key is found.
+         */
+        NONE;
+    }
+}