Add `DefaultDeserializer.getClassLoader()`

[artembilan]

In Spring Integration we rely on the ClassLoader from the provided DefaultDeserializer and use for the purpose reflection:

	public AllowListDeserializingConverter(Deserializer<Object> deserializer) {
		Assert.notNull(deserializer, "Deserializer must not be null");
		this.deserializer = deserializer;
		if (deserializer instanceof DefaultDeserializer) {
			ClassLoader classLoader = null;
			try {
				classLoader = (ClassLoader) new DirectFieldAccessor(deserializer).getPropertyValue("classLoader");
			}
			catch (Exception e) {
				// no-op
			}
			this.defaultDeserializerClassLoader = classLoader;
			this.usingDefaultDeserializer = true;
		}
		else {
			this.defaultDeserializerClassLoader = null;
			this.usingDefaultDeserializer = false;
		}
	}

That defaultDeserializerClassLoader is used afterwards for an internal ConfigurableObjectInputStream extension:

			ObjectInputStream objectInputStream = new ConfigurableObjectInputStream(inputStream,
					this.defaultDeserializerClassLoader) {

				@Override
				protected Class<?> resolveClass(ObjectStreamClass classDesc)
						throws IOException, ClassNotFoundException {
					Class<?> clazz = super.resolveClass(classDesc);
					checkAllowList(clazz);
					return clazz;
				}

			};

If you expose a DefaultDeserializer.getClassLoader(), it would be very helpful in our case to avoid reflection and possible native image hint.

Thanks