Support an alternative fat jar format that is more amenable to Docker
image layers.
The new format arranges files in the following structure:
BOOT-INF/
layers/
<layer-name #1>
/classes
/lib
<layer-name #2>
/classes
/lib
The `BOOT-INF/layers.idx` file provides the names of the layers and the
order in which they should be added (starting with the least changed).
The `JarLauncher` class can load layered jars in both fat and exploded
forms.
Closes gh-19767
Co-authored-by: Phillip Webb <pwebb@pivotal.io>
e9d61bac75