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/Optional.java

@@ -0,0 +1,61 @@
+/*
+ * 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;
+
+/**
+ * Backport of java.util.Optional for older Android versions.
+ *
+ * @param <T> item type
+ */
+public class Optional<T> {
+
+    private final T item;
+
+    public Optional() {
+        this(null);
+    }
+
+    public Optional(T item) {
+        this.item = item;
+    }
+
+    public static <T> Optional<T> of(T item) {
+        if (item == null) {
+            throw new NullPointerException("Item cannot be null.");
+        }
+        return new Optional<>(item);
+    }
+
+    public static <T> Optional<T> ofNullable(T item) {
+        return new Optional<>(item);
+    }
+
+    public static <T> Optional<T> ofEmpty() {
+        return new Optional<>(null);
+    }
+
+    public T get() {
+        return item;
+    }
+
+    public boolean isPresent() {
+        return item != null;
+    }
+
+    public boolean isEmpty() {
+        return item == null;
+    }
+}