Allow ApplicationContextAotGenerator to generated better class names

[philwebb]

Currently ApplicationContextAotGenerator always generates a __.BeanFactoryRegistrations class. For context hierarchy support we're going to need multiple registration classes so we should use different names. We'd also like to be able to provide a target class so that we can use a package other than __.

[snicoll]

The default classname is still generated in that __ package. Is that expected? I was under the impression that this issue would move it at the same level as the application class itself.

[philwebb]

We'll need to make some updates on the Boot side to pass the target in.