[martinaldrin]

Proxy can now generate its proxies with either Javassist or Byte Buddy. Javassist stays the default and its code path is unchanged.

Select the engine with -Dproxy.engine=javassist|bytebuddy, or at runtime with ProxyConfiguration.setEngine/setThreadEngine/callWithEngine.

The public API is untouched: no existing test file was modified, and the whole suite now runs twice, once per engine, from a second surefire execution. 222 tests pass on both engines on JDK 11, 17, 21 and 26.

Design notes:

• The engine is behind a ProxyEngineFactory SPI, resolved lazily by ProxyEngineProvider, so byte-buddy classes are never loaded unless the byte-buddy engine is actually selected.
• The earlier attempt at byte-buddy support used InvocationHandlerAdapter, which cannot reach the original implementation and therefore broke Invocation.invoke(). This uses MethodDelegation with @supermethod(nullIfImpossible = true) instead, which is the direct equivalent of javassist's proceed method.
• Proxy classes are named into the proxied type's package and defined via MethodHandles.privateLookupIn, so package private types, interfaces and methods keep working. Class injection is only a fallback.
• The per instance interceptor stack lives in a private synthetic field. It must not be public, or Util.isClassSafeFromPublicVariableProblems would silently refuse mdcLogging and recursiveIntercept on a proxy.
• Generated classes are shared through a byte-buddy TypeCache keyed on type names only. Without it every proxy instance got its own class: 5000 proxies cost 3219 loaded classes, now it is 1218.