Completion command for bash

- Add basic support of defining a command `completion bash` which
  outputs a generic bash script which can be used in a user environment.
- Idea for completion is copied from go's cobra library what comes for
  a bash dance itself.
- Goes through command registry, builds a model for command structure
  and uses antlr st4 for templating bash.
- Should give foundation to create other completions just like in cobra.
- Currently as we don't know a root-command in a generic way, option
  `spring.shell.command.completion.root-command` is required user to set.
- Fixes #343

spring-shell-standard-commands/src/main/java/org/springframework/shell/standard/commands/Completion.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2022 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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 org.springframework.shell.standard.commands;
+
+import java.util.stream.Collectors;
+
+import org.springframework.context.ResourceLoaderAware;
+import org.springframework.core.io.ResourceLoader;
+import org.springframework.shell.standard.AbstractShellComponent;
+import org.springframework.shell.standard.ShellComponent;
+import org.springframework.shell.standard.ShellMethod;
+import org.springframework.shell.standard.completion.BashCompletions;
+
+/**
+ * Command to create a shell completion files, i.e. for {@code bash}.
+ *
+ * @author Janne Valkealahti
+ */
+@ShellComponent
+public class Completion extends AbstractShellComponent implements ResourceLoaderAware {
+
+	/**
+	 * Marker interface used in auto-config.
+	 */
+	public interface Command {
+	}
+
+	private ResourceLoader resourceLoader;
+	private String rootCommand;
+
+	public Completion(String rootCommand) {
+		this.rootCommand = rootCommand;
+	}
+
+	@Override
+	public void setResourceLoader(ResourceLoader resourceLoader) {
+		this.resourceLoader = resourceLoader;
+	}
+
+	@ShellMethod(key = "completion bash", value = "Generate bash completion script")
+	public String bash() {
+		BashCompletions bashCompletions = new BashCompletions(resourceLoader, getCommandRegistry(),
+				getParameterResolver().collect(Collectors.toList()));
+		return bashCompletions.generate(rootCommand);
+	}
+}