Allow Cura front-end to set plugins (port & address)

This can currently be done by setting the environment variables:
- SIMPLIFY_ENABLE: default disabled, setting this to any value will enable the plugin
- SIMPLIFY_ADDRESS: defaults to localhost
- SIMPLIFY_PORT: defaults to 33700
- POSTPROCESS_ENABLE: default disabled, setting this to any value will enable the plugin
- POSTPROCESS_ADDRESS: defaults to localhost
- POSTPROCESS_PORT: defaults to 33701

Hacky for now

Contributes to CURA-10475

plugins/CuraEngineBackend/Cura.proto

@@ -8,12 +8,25 @@ message ObjectList
     repeated Setting settings = 2; // meshgroup settings (for one-at-a-time printing)
 }
 
+enum SlotID {
+  SIMPLIFY = 0;
+  POSTPROCESS = 1;
+}
+
+message EnginePlugin
+{
+    SlotID id = 1;
+    optional string address = 2;
+    optional uint32 port = 3;
+}
+
 message Slice
 {
     repeated ObjectList object_lists = 1; // The meshgroups to be printed one after another
     SettingList global_settings = 2; // The global settings used for the whole print job
     repeated Extruder extruders = 3; // The settings sent to each extruder object
     repeated SettingExtruder limit_to_extruder = 4; // From which stack the setting would inherit if not defined per object
+    repeated EnginePlugin engine_plugins = 5;
 }
 
 message Extruder

plugins/CuraEngineBackend/StartSliceJob.py

@@ -1,5 +1,6 @@
 #  Copyright (c) 2021-2022 Ultimaker B.V.
 #  Cura is released under the terms of the LGPLv3 or higher.
+import os
 
 import numpy
 from string import Formatter
@@ -301,6 +302,22 @@ class StartSliceJob(Job):
         for extruder_stack in global_stack.extruderList:
             self._buildExtruderMessage(extruder_stack)
 
+        # EnginePlugins
+        # TODO: don't hardcode them
+        # Ports: are chosen based on https://stackoverflow.com/questions/10476987/best-tcp-port-number-range-for-internal-applications
+
+        plugins = {
+            0: {"address": os.environ.get("SIMPLIFY_ADDRESS", "localhost"), "port": os.environ.get("SIMPLIFY_PORT", 33700)} if os.environ.get("SIMPLIFY_ENABLE") is not None else None,
+            1: {"address": os.environ.get("POSTPROCESS_ADDRESS", "localhost"), "port": os.environ.get("POSTPROCESS_PORT", 33701)} if os.environ.get("POSTPROCESS_ENABLE") is not None else None,
+        }
+
+        for plugin, connection in plugins.items():
+            plugin_message = self._slice_message.addRepeatedMessage("engine_plugins")
+            plugin_message.id = plugin
+            if connection:
+                plugin_message.address = connection["address"]
+                plugin_message.port = connection["port"]
+
         for group in filtered_object_groups:
             group_message = self._slice_message.addRepeatedMessage("object_lists")
             parent = group[0].getParent()