Enable to upgrade proxy generation independently of beam for java support

Beam is now using a custom shaded version of bytebudy which makes impossible - until you reshade - to upgrade bytebuddy without requiring a new beam release.

However with the fast release rate of the JVM it is important to be able to upgrade bytebuddy - at least while beam is using it which is technically not a strong requirement - to enable to run on the new JVM.

For example, last beam release does not support recent java:

Caused by: java.lang.UnsupportedOperationException: Cannot define class using reflection: Cannot define nest member class java.lang.reflect.AccessibleObject$Cache + within different package then class org.apache.beam.repackaged.beam_sdks_java_core.net.bytebuddy.mirror.AccessibleObject

My preference to fix this issue would be to relax the proxying definition to just use a "proxy classloader" where the proxy would be defined but it requires to be able to attach it to an execution - where beam is not yet super clean.
Alternative is to have a SPI for the asm usage and enable to user to replace the bytebuddy impl with either a not shaded version or even a pure asm one to let him control the dependencies.

Romain