[cli] new 'author template' command (#6441)

* [cli] new 'author template' command

This new command allows users to extract templates for authoring
(customization) without the complexity of finding and downloading a
specific directory for their versioned artifact.

Example usage:

```
openapi-generator author template -g java --library webclient
```

This will write all templates with library-specific templates to the
'./out' directory relative to the current directory.

CLI will refer the user to
https://openapi-generator.tech/docs/templating after generation

* [docs] Usage of author template command

* Log warning if author template fails to output requested library

modules/openapi-generator-cli/src/test/java/org/openapitools/codegen/cmd/AuthorTemplateTest.java

@@ -0,0 +1,71 @@
+package org.openapitools.codegen.cmd;
+
+import io.airlift.airline.Cli;
+import org.testng.Assert;
+import org.testng.ITestContext;
+import org.testng.annotations.BeforeTest;
+import org.testng.annotations.Test;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Arrays;
+
+public class AuthorTemplateTest {
+    Path outputDirectory;
+
+    @BeforeTest
+    public void setUp(ITestContext ctx) throws IOException {
+        outputDirectory = Files.createTempDirectory("AuthorTemplateTest");
+        outputDirectory.toFile().deleteOnExit();
+    }
+
+    @Test
+    public void smokeTestAuthorTemplateCommand(){
+        Cli.CliBuilder<Runnable> builder = createBuilder();
+        String[] arguments = new String[]{
+                "author",
+                "template",
+                "-g",
+                "java",
+                "--library",
+                "webclient",
+                "--output",
+                outputDirectory.toAbsolutePath().toString()
+        };
+        builder.build().parse(arguments).run();
+
+        // spot check root files
+        Assert.assertTrue(Files.exists(outputDirectory.resolve("ApiClient.mustache")));
+        Assert.assertTrue(Files.exists(outputDirectory.resolve("api_doc.mustache")));
+        Assert.assertTrue(Files.exists(outputDirectory.resolve("pom.mustache")));
+
+        Assert.assertTrue(Files.exists(outputDirectory.resolve("auth/OAuth.mustache")));
+
+        // check libraries files and subdirectories
+        Assert.assertTrue(Files.exists(outputDirectory.resolve("libraries/webclient/ApiClient.mustache")));
+        Assert.assertTrue(Files.exists(outputDirectory.resolve("libraries/webclient/pom.mustache")));
+        Assert.assertTrue(Files.exists(outputDirectory.resolve("libraries/webclient/auth/OAuth.mustache")));
+
+        // check non-existence of unselected libraries
+        Assert.assertFalse(Files.exists(outputDirectory.resolve("libraries/feign/build.gradle.mustache")));
+        Assert.assertFalse(Files.exists(outputDirectory.resolve("libraries/feign/auth/OAuth.mustache")));
+
+        Assert.assertFalse(Files.exists(outputDirectory.resolve("libraries/jersey2/api_doc.mustache")));
+        Assert.assertFalse(Files.exists(outputDirectory.resolve("libraries/jersey2/auth/HttpBasicAuth.mustache")));
+
+        Assert.assertFalse(Files.exists(outputDirectory.resolve("libraries/okhttp-gson/api.mustache")));
+        Assert.assertFalse(Files.exists(outputDirectory.resolve("libraries/okhttp-gson/auth/RetryingOAuth.mustache")));
+    }
+
+    private Cli.CliBuilder<Runnable> createBuilder(){
+        Cli.CliBuilder<Runnable> builder = new Cli.CliBuilder<>("openapi-generator-cli");
+
+        builder.withGroup("author")
+                .withDescription("Utilities for authoring generators or customizing templates.")
+                .withDefaultCommand(HelpCommand.class)
+                .withCommands(AuthorTemplate.class);
+
+        return builder;
+    }
+}