Fix OOM when detached signing large files

Fixes #432

pgpainless-sop/src/main/java/org/pgpainless/sop/DetachedSignImpl.java

@@ -4,7 +4,6 @@
 
 package org.pgpainless.sop;
 
-import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
@@ -98,10 +97,10 @@ public class DetachedSignImpl implements DetachedSign {
             }
         }
 
-        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
+        OutputStream sink = new NullOutputStream();
         try {
             EncryptionStream signingStream = PGPainless.encryptAndOrSign()
-                    .onOutputStream(buffer)
+                    .onOutputStream(sink)
                     .withOptions(ProducerOptions.sign(signingOptions)
                             .setAsciiArmor(armor));
 

pgpainless-sop/src/main/java/org/pgpainless/sop/NullOutputStream.java

@@ -0,0 +1,17 @@
+// SPDX-FileCopyrightText: 2024 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.sop;
+
+import java.io.OutputStream;
+
+/**
+ * {@link OutputStream} that simply discards bytes written to it.
+ */
+public class NullOutputStream extends OutputStream {
+    @Override
+    public void write(int b) {
+        // NOP
+    }
+}