Fix availability feature

- Bring back some missing functionality which got missing
  during the rework to new command model.
- Polish some classes.
- Restore origin sample.
- Add availability things into help templates and its
  representation model.
- Fixes #423

spring-shell-core/src/main/java/org/springframework/shell/CommandNotCurrentlyAvailable.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017 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.
@@ -13,7 +13,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
 package org.springframework.shell;
 
 /**
@@ -23,20 +22,21 @@ package org.springframework.shell;
  */
 public class CommandNotCurrentlyAvailable extends RuntimeException {
 
-    private final String command;
-    private final Availability availability;
+	private final String command;
+	private final Availability availability;
 
-    public CommandNotCurrentlyAvailable(String command, Availability availability) {
-        super(String.format("Command '%s' exists but is not currently available because %s", command, availability.getReason()));
-        this.command = command;
-        this.availability = availability;
-    }
+	public CommandNotCurrentlyAvailable(String command, Availability availability) {
+		super(String.format("Command '%s' exists but is not currently available because %s", command,
+				availability.getReason()));
+		this.command = command;
+		this.availability = availability;
+	}
 
-    public String getCommand() {
-        return command;
-    }
+	public String getCommand() {
+		return command;
+	}
 
-    public Availability getAvailability() {
-        return availability;
-    }
+	public Availability getAvailability() {
+		return availability;
+	}
 }

spring-shell-core/src/main/java/org/springframework/shell/command/CommandExecution.java

@@ -29,6 +29,8 @@ import org.springframework.core.convert.ConversionService;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.handler.invocation.HandlerMethodArgumentResolver;
 import org.springframework.messaging.support.MessageBuilder;
+import org.springframework.shell.Availability;
+import org.springframework.shell.CommandNotCurrentlyAvailable;
 import org.springframework.shell.command.CommandParser.CommandParserException;
 import org.springframework.shell.command.CommandParser.CommandParserResults;
 import org.springframework.shell.command.CommandRegistration.TargetInfo;
@@ -95,6 +97,11 @@ public interface CommandExecution {
 		}
 
 		public Object evaluate(CommandRegistration registration, String[] args) {
+			// fast fail with availability before doing anything else
+			Availability availability = registration.getAvailability();
+			if (availability != null && !availability.isAvailable()) {
+				return new CommandNotCurrentlyAvailable(registration.getCommand(), availability);
+			}
 
 			List<CommandOption> options = registration.getOptions();
 			CommandParser parser = CommandParser.of(conversionService);