Separate user code classloader from NK GUI classloader

[mproch]

https://groups.google.com/forum/#!msg/nussknacker/z2IH8UbPojM/VHaU-YgCAQAJ - problem described by user.
Probably something like https://ci.apache.org/projects/flink/flink-docs-stable/ops/plugins.html will be needed to better separate model classes from system classloader?

[mproch]

https://github.com/apache/flink/blob/master/flink-core/src/main/java/org/apache/flink/core/plugin/PluginLoader.java#L114 => this is relevant classloader in Flink. Probably we can proceed more or less like this:

• each ProcessManager has to be separate "plugin"
• model defines further child classloader
• communication with the rest of NK is done via packages available on allowList.
  In the case of Flink, ProcessManager would contain all relevant Flink code.