Port some missing samples. Add a method to compute user password

Author: ars18wrw

core/src/main/java/com/itextpdf/core/pdf/PdfReader.java

@@ -1318,4 +1318,12 @@ public void close() throws IOException {
             buffer = null;
         }
     }
+
+    /**
+     * @return byte array of computed user password, or null if not encrypted or no ownerPassword is used.
+     */
+    public byte[] computeUserPassword() {
+        if (!encrypted || !ownerPasswordUsed) return null;
+        return decrypt.computeUserPassword(password);
+    }
 }