Base PGPainlessCLI on new sop-java module

* Rename pgpainless-sop -> pgpainless-cli
* Introduce sop-java (implementation-independent SOP API)
* Introduce sop-java-picocli (CLI frontend for sop-java)
* Introduce pgpainless-sop (implementation of sop-java using PGPainless)
* Rework pgpainless-cli (plugs pgpainless-sop into sop-java-picocli)

sop-java/src/main/java/sop/util/ProxyOutputStream.java

@@ -0,0 +1,91 @@
+/*
+ * Copyright 2021 Paul Schaub.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package sop.util;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.OutputStream;
+
+/**
+ * {@link OutputStream} that buffers data being written into it, until its underlying output stream is being replaced.
+ * At that point, first all the buffered data is being written to the underlying stream, followed by any successive
+ * data that may get written to the {@link ProxyOutputStream}.
+ *
+ * This class is useful if we need to provide an {@link OutputStream} at one point in time where the final
+ * target output stream is not yet known.
+ */
+public class ProxyOutputStream extends OutputStream {
+
+    private final ByteArrayOutputStream buffer;
+    private OutputStream swapped;
+
+    public ProxyOutputStream() {
+        this.buffer = new ByteArrayOutputStream();
+    }
+
+    public synchronized void replaceOutputStream(OutputStream underlying) throws IOException {
+        if (underlying == null) {
+            throw new NullPointerException("Underlying OutputStream cannot be null.");
+        }
+        this.swapped = underlying;
+
+        byte[] bufferBytes = buffer.toByteArray();
+        swapped.write(bufferBytes);
+    }
+
+    @Override
+    public synchronized void write(byte[] b) throws IOException {
+        if (swapped == null) {
+            buffer.write(b);
+        } else {
+            swapped.write(b);
+        }
+    }
+
+    @Override
+    public synchronized void write(byte[] b, int off, int len) throws IOException {
+        if (swapped == null) {
+            buffer.write(b, off, len);
+        } else {
+            swapped.write(b, off, len);
+        }
+    }
+
+    @Override
+    public synchronized void flush() throws IOException {
+        buffer.flush();
+        if (swapped != null) {
+            swapped.flush();
+        }
+    }
+
+    @Override
+    public synchronized void close() throws IOException {
+        buffer.close();
+        if (swapped != null) {
+            swapped.close();
+        }
+    }
+
+    @Override
+    public synchronized void write(int i) throws IOException {
+        if (swapped == null) {
+            buffer.write(i);
+        } else {
+            swapped.write(i);
+        }
+    }
+}