Add layertools jarmode

Add a new `spring-boot-layertools` module which provides jarmode support
for working with layers. The module works with both classic fat jars,
as well as layered jars.

Closes gh-19849

spring-boot-project/spring-boot-dependencies/build.gradle

@@ -1425,6 +1425,7 @@ bom {
 				'spring-boot-configuration-metadata',
 				'spring-boot-configuration-processor',
 				'spring-boot-devtools',
+				'spring-boot-layertools',
 				'spring-boot-loader',
 				'spring-boot-loader-tools',
 				'spring-boot-properties-migrator',