Fall back from cglib to JDK proxies when possible

I am using Spring Boot 2.7, which defaults to cglib proxies. I have a JavaConfig bean definition:

interface ServiceInterface {
  @Async void log()
}

@Bean
ServiceInterface myBean() {
  new Impl()
}

If the Impl class is not subclassable by cglib (in this case, it uses a static factory method and has a private constructor), context startup fails with a "cglib can't subclass" error. However, if the declared bean type is an interface, in my view the ProxyFactory ought to use a JDK proxy instead (in the first place, but certainly if cglib fails).

Start at ProxyFactory and reproduce the JavaConfig case described in the issue, where the declared bean type is an interface but the implementation cannot be subclassed by cglib. Trace how proxy selection handles that failure and add a regression test showing that a JDK proxy is used when possible; done means context startup succeeds for this configuration.