Design and implement a layered jar format

[philwebb]

Following on from #12545 we'd like to enhance our fat jar format to support layer folders that are easier to use when writing Dockerfiles.

The following issues will need to be addressed:

• Polish loader and loader-tools to make it easier to add new features (Polish loader and loader-tools to make it easier to add new features #19766)
• Design a new fat jar format that can support layers in a logical way
• • Decide what to do about the Spring-Boot-Lib attribute
• • Provide a manifest attribute that points to the layers index
• • Provide a manifest attribute that points to the classpath index
• • Provide an implicit layer for buildpack contributions (Provide an implicit layer for buildpack contributions #19771) (target after M1)
• Update the Repackager and launcher to support the new format (Support generation and loading of layered jars #19767)
• Update the Maven plugin with an option to generate the new format ( Update the Maven plugin with an option to generate layered jars #19768)
• Update the Maven plugin to allow layer customization (Provide a way to customize layers with the Maven plugin #20295)
• Update the Gradle plugin with an option to generate the new format ( Update the Gradle plugin with an option to generate layered jars #19792)
• Update the Gradle plugin to allow layer customization (Provide a way to customize layers with the Gradle plugin #20296)
• Add jarmode support to the loader code (Add jarmode support to the loader code #19848)
• Add layertools jarmode (Add layertools jarmode #19849)
• Automatically add jarmode jars when packaging (Automatically add jarmode jars when packaging #19865)
• Add layertools jarmode support to Maven ( Allow users to opt out of including the layer tools in a layered jar #19866)
• Add layertools jarmode support to Gradle ( Add layertools jarmode support to Gradle #19867)
• Provide (or generate) a Dockerfile showing how layered jars can be converted to images
• Document the new format somewhere
• Raise a java buildpack issue to ensure the new format can work with the buildpack (Add support for a layered Spring Boot jar cloudfoundry/spring-boot-cnb#6 and Update Spring Boot layered jar support cloudfoundry/spring-boot-cnb#9)

[philwebb]

Decide what to do about the Spring-Boot-Lib attribute

Since there isn't one I've dropped it when creating a layered jar. There's a new Spring-Boot-Layers-Index attribute that points to the index file. The buildpack could use this to detect the new format.