Add support for a layered Spring Boot jar

[mbhave]

Spring Boot 2.3.0.M1 introduced support for building layered jars. The structure for the jar is as follows:

META-INF/
  MANIFEST.MF
org/
  springframework/
    boot/
      loader/
        ...
BOOT-INF/
  layers/
    <name>/
      classes/
        ...
      lib/
        ...
    <name>/
      classes/
        ...
      lib/
        ...
  layers.idx

At the moment the layers are quite similar to what the buildpack does by default but we plan on adding support for layer customization. The buildpack can look at the layers.idx to figure out what the layers in the image are rather than using the defaults.