Add dynamic command registration

- Extend CommandRegistry for add/remove methods.
- For rest of shell classes move to use registry
  directly instead of caching commands as registry
  is not immutable anymore.
- Add new sample
- Fixes #379

spring-shell-samples/src/main/java/org/springframework/shell/samples/standard/RegisterCommands.java

@@ -0,0 +1,65 @@
+/*
+ * 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.samples.standard;
+
+import org.springframework.shell.MethodTarget;
+import org.springframework.shell.standard.AbstractShellComponent;
+import org.springframework.shell.standard.ShellComponent;
+import org.springframework.shell.standard.ShellMethod;
+import org.springframework.shell.standard.ShellOption;
+
+@ShellComponent
+public class RegisterCommands extends AbstractShellComponent {
+
+	private final PojoMethods pojoMethods = new PojoMethods();
+
+    @ShellMethod(key = "register add", value = "Register commands", group = "Register Commands")
+    public String register() {
+		MethodTarget target1 = MethodTarget.of("dynamic1", pojoMethods, "Dynamic1 command", "Register Commands");
+		MethodTarget target2 = MethodTarget.of("dynamic2", pojoMethods, "Dynamic2 command", "Register Commands");
+		MethodTarget target3 = MethodTarget.of("dynamic3", pojoMethods, "Dynamic3 command", "Register Commands");
+		getCommandRegistry().addCommand("register dynamic1", target1);
+		getCommandRegistry().addCommand("register dynamic2", target2);
+		getCommandRegistry().addCommand("register dynamic3", target3);
+		return "Registered commands dynamic1, dynamic2, dynamic3";
+    }
+
+    @ShellMethod(key = "register remove", value = "Deregister commands", group = "Register Commands")
+    public String deregister() {
+		getCommandRegistry().removeCommand("register dynamic1");
+		getCommandRegistry().removeCommand("register dynamic2");
+		getCommandRegistry().removeCommand("register dynamic3");
+		return "Deregistered commands dynamic1, dynamic2, dynamic3";
+    }
+
+	public static class PojoMethods {
+
+		@ShellMethod
+		public String dynamic1() {
+			return "dynamic1";
+		}
+
+		@ShellMethod
+		public String dynamic2(String arg1) {
+			return "dynamic2" + arg1;
+		}
+
+		@ShellMethod
+		public String dynamic3(@ShellOption(defaultValue = ShellOption.NULL) String arg1) {
+			return "dynamic3" + arg1;
+		}
+	}
+}