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-picocli/src/main/java/sop/cli/picocli/SopCLI.java

@@ -0,0 +1,71 @@
+/*
+ * 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.cli.picocli;
+
+import picocli.CommandLine;
+import sop.SOP;
+import sop.cli.picocli.commands.ArmorCmd;
+import sop.cli.picocli.commands.DearmorCmd;
+import sop.cli.picocli.commands.DecryptCmd;
+import sop.cli.picocli.commands.EncryptCmd;
+import sop.cli.picocli.commands.ExtractCertCmd;
+import sop.cli.picocli.commands.GenerateKeyCmd;
+import sop.cli.picocli.commands.SignCmd;
+import sop.cli.picocli.commands.VerifyCmd;
+import sop.cli.picocli.commands.VersionCmd;
+
+@CommandLine.Command(
+        exitCodeOnInvalidInput = 69,
+        subcommands = {
+                ArmorCmd.class,
+                DearmorCmd.class,
+                DecryptCmd.class,
+                EncryptCmd.class,
+                ExtractCertCmd.class,
+                GenerateKeyCmd.class,
+                SignCmd.class,
+                VerifyCmd.class,
+                VersionCmd.class
+        }
+)
+public class SopCLI {
+
+    static SOP SOP_INSTANCE;
+
+    public static void main(String[] args) {
+        int exitCode = execute(args);
+        if (exitCode != 0) {
+            System.exit(exitCode);
+        }
+    }
+
+    public static int execute(String[] args) {
+        return new CommandLine(SopCLI.class)
+                .setCaseInsensitiveEnumValuesAllowed(true)
+                .execute(args);
+    }
+
+    public static SOP getSop() {
+        if (SOP_INSTANCE == null) {
+            throw new IllegalStateException("No SOP backend set.");
+        }
+        return SOP_INSTANCE;
+    }
+
+    public static void setSopInstance(SOP instance) {
+        SOP_INSTANCE = instance;
+    }
+}