Allow users to opt out of including the layer tools in a layered jar

For Maven, the layer configuration is now an additional configuration
option instead of a layout type.

Closes gh-19866

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/packaging.adoc

@@ -287,3 +287,18 @@ The jar will then be split into layer folders which may include:
 * `snapshots-dependencies`
 * `dependencies`
 
+When you create a layered jar, the `spring-boot-layertools` jar will be added as a dependency to your jar.
+With this jar on the classpath, you can launch your application in a special mode which allows the bootstrap code to run something entirely different from your application, for example, something that extracts the layers.
+If you wish to exclude this dependency, you can do so in the following manner:
+
+[source,groovy,indent=0,subs="verbatim,attributes",role="primary"]
+.Groovy
+----
+include::../gradle/packaging/boot-jar-layered-exclude-tools.gradle[tags=layered]
+----
+
+[source,kotlin,indent=0,subs="verbatim,attributes",role="secondary"]
+.Kotlin
+----
+include::../gradle/packaging/boot-jar-layered-exclude-tools.gradle.kts[tags=layered]
+----

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-layered-exclude-tools.gradle

@@ -0,0 +1,16 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+bootJar {
+	mainClassName 'com.example.ExampleApplication'
+}
+
+// tag::layered[]
+bootJar {
+	layered {
+		includeLayerTools = false
+	}
+}
+// end::layered[]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-layered-exclude-tools.gradle.kts

@@ -0,0 +1,18 @@
+import org.springframework.boot.gradle.tasks.bundling.BootJar
+
+plugins {
+	java
+	id("org.springframework.boot") version "{version}"
+}
+
+tasks.getByName<BootJar>("bootJar") {
+	mainClassName = "com.example.ExampleApplication"
+}
+
+// tag::layered[]
+tasks.getByName<BootJar>("bootJar") {
+	layered {
+		includeLayerTools = false
+	}
+}
+// end::layered[]