Support hidden commands

- CommandRegistration now has a structure to define
  it beind hidden
- Modify relevant parts to filter out hidden commands
- Essentially command is hidden from all other than
  command execution
- Sample in e2e tests
- Fixes #416

spring-shell-docs/src/main/asciidoc/using-shell-commands-hidden.adoc

@@ -0,0 +1,27 @@
+[[commands-hidden]]
+=== Hidden Command
+ifndef::snippets[:snippets: ../../test/java/org/springframework/shell/docs]
+
+It is possible to _hide_ a command which is convenient in cases where it is not yet ready for
+prime time, is meant for debugging purposes or you have any other reason you dont want to
+advertise its presense.
+
+Hidden command can be executed if you know it and its options. It is effectively removed
+from:
+
+* Help listing
+* Help page for command return "unknown command"
+* Command completion in interactive mode
+* Bash completion
+
+Below is an example how to define command as _hidden_. It shows available builder methods
+to define _hidden_ state.
+
+====
+[source, java, indent=0]
+----
+include::{snippets}/CommandRegistrationHiddenSnippets.java[tag=snippet1]
+----
+====
+
+NOTE: Defining hidden commands is not supported with annotation based configuration

spring-shell-docs/src/main/asciidoc/using-shell-commands.adoc

@@ -20,3 +20,5 @@ include::using-shell-commands-organize.adoc[]
 include::using-shell-commands-availability.adoc[]
 
 include::using-shell-commands-exceptionhandling.adoc[]
+
+include::using-shell-commands-hidden.adoc[]

spring-shell-docs/src/test/java/org/springframework/shell/docs/CommandRegistrationHiddenSnippets.java

@@ -0,0 +1,36 @@
+/*
+ * 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.docs;
+
+import org.springframework.shell.command.CommandRegistration;
+
+class CommandRegistrationHiddenSnippets {
+
+	// tag::snippet1[]
+	CommandRegistration commandRegistration() {
+		return CommandRegistration.builder()
+			.command("mycommand")
+			// define as hidden
+			.hidden()
+			// can be defined via a flag (false)
+			.hidden(false)
+			// can be defined via a flag (true)
+			.hidden(true)
+			.build();
+	}
+	// end::snippet1[]
+
+}