springboot Support generation and loading of layered jars

As part of #19697 we need to update the Repackager to generate layered JARs and update the JarLauncher to support them.

Comment From: wilkinsona

Re-opening as I think we need some equivalent Gradle changes and I don't want them to fall through the cracks.

Comment From: philwebb

We'll handle the Gradle work in #19792

Comment From: philwebb

I messed up and managed to not actually write the index file.