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-autoconfigure/src/main/java/org/springframework/shell/boot/SpringShellProperties.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2021 the original author or authors.
+ * Copyright 2021-2022 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.
@@ -179,6 +179,28 @@ public class SpringShellProperties {
 		}
 	}
 
+	public static class CompletionCommand {
+
+		private boolean enabled = true;
+		private String rootCommand;
+
+		public boolean isEnabled() {
+			return enabled;
+		}
+
+		public void setEnabled(boolean enabled) {
+			this.enabled = enabled;
+		}
+
+		public String getRootCommand() {
+			return rootCommand;
+		}
+
+		public void setRootCommand(String rootCommand) {
+			this.rootCommand = rootCommand;
+		}
+	}
+
 	public static class Command {
 
 		private HelpCommand help = new HelpCommand();
@@ -187,6 +209,7 @@ public class SpringShellProperties {
 		private StacktraceCommand stacktrace = new StacktraceCommand();
 		private ScriptCommand script = new ScriptCommand();
 		private HistoryCommand history = new HistoryCommand();
+		private CompletionCommand completion = new CompletionCommand();
 
 		public void setHelp(HelpCommand help) {
 			this.help = help;
@@ -235,5 +258,13 @@ public class SpringShellProperties {
 		public void setHistory(HistoryCommand history) {
 			this.history = history;
 		}
+
+		public CompletionCommand getCompletion() {
+			return completion;
+		}
+
+		public void setCompletion(CompletionCommand completion) {
+			this.completion = completion;
+		}
 	}
 }

spring-shell-autoconfigure/src/main/java/org/springframework/shell/boot/StandardCommandsAutoConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2021 the original author or authors.
+ * Copyright 2017-2022 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.
@@ -22,10 +22,14 @@ import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Conditional;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.shell.boot.condition.OnCompletionCommandCondition;
 import org.springframework.shell.result.ThrowableResultHandler;
 import org.springframework.shell.standard.commands.Clear;
+import org.springframework.shell.standard.commands.Completion;
 import org.springframework.shell.standard.commands.Help;
 import org.springframework.shell.standard.commands.History;
 import org.springframework.shell.standard.commands.Quit;
@@ -39,6 +43,7 @@ import org.springframework.shell.standard.commands.Stacktrace;
  */
 @Configuration(proxyBeanMethods = false)
 @ConditionalOnClass({ Help.Command.class })
+@EnableConfigurationProperties(SpringShellProperties.class)
 public class StandardCommandsAutoConfiguration {
 
 	@Bean
@@ -82,4 +87,11 @@ public class StandardCommandsAutoConfiguration {
 	public History historyCommand(org.jline.reader.History jLineHistory) {
 		return new History(jLineHistory);
 	}
+
+	@Bean
+	@ConditionalOnMissingBean(Completion.Command.class)
+	@Conditional(OnCompletionCommandCondition.class)
+	public Completion completion(SpringShellProperties properties) {
+		return new Completion(properties.getCommand().getCompletion().getRootCommand());
+	}
 }

spring-shell-autoconfigure/src/main/java/org/springframework/shell/boot/condition/OnCompletionCommandCondition.java

@@ -0,0 +1,34 @@
+/*
+ * 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.boot.condition;
+
+import org.springframework.boot.autoconfigure.condition.AllNestedConditions;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+
+public class OnCompletionCommandCondition extends AllNestedConditions {
+
+	public OnCompletionCommandCondition() {
+		super(ConfigurationPhase.REGISTER_BEAN);
+	}
+
+	@ConditionalOnProperty(prefix = "spring.shell.command.completion", value = "root-command")
+	static class RootNameCondition {
+	}
+
+	@ConditionalOnProperty(prefix = "spring.shell.command.completion", value = "enabled", havingValue = "true", matchIfMissing = true)
+	static class EnabledCondition {
+	}
+}