[Enhencement](Export) add property for outfile/export and add test (#18997)

This pr does three things:
1. add `delete_existing_files` property for outfile/export. If `delete_existing_files = true`, export/outfile will delete all files under file_path first.
2. add p2 test for export
3. modify docs

fe/fe-common/src/main/java/org/apache/doris/common/Config.java

@@ -1923,5 +1923,13 @@ public class Config extends ConfigBase {
 
     @ConfField(mutable = true)
     public static boolean disable_datev1  = true;
+
+    /**
+     * This config used for export/outfile.
+     * Whether delete all files in the directory specified by export/outfile.
+     * It is a very dangerous operation, should only be used in test env.
+     */
+    @ConfField(mutable = false)
+    public static boolean enable_delete_existing_files  = false;
 }