Move backend plugin logic to Cura from Uranium

I was running into abstraction issues when it was defined in Uranium.
Instead of trying to fight those, it's just easier to move it to Cura

CURA-10717

cura/BackendPlugin.py

@@ -0,0 +1,74 @@
+# Copyright (c) 2023 Ultimaker B.V.
+# Cura is released under the terms of the LGPLv3 or higher.
+import subprocess
+from typing import Optional, List
+
+from UM.Logger import Logger
+from UM.PluginObject import PluginObject
+
+
+class BackendPlugin(PluginObject):
+    def __init__(self) -> None:
+        super().__init__()
+        self.__port: int = 0
+        self._plugin_address: str = "127.0.0.1"
+        self._plugin_command: Optional[List[str]] = None
+        self._process = None
+        self._is_running = False
+
+    def isRunning(self):
+        return self._is_running
+
+    def setPort(self, port: int) -> None:
+        self.__port = port
+
+    def getPort(self) -> int:
+        return self.__port
+
+    def _validatePluginCommand(self) -> list[str]:
+        """
+        Validate the plugin command and add the port parameter if it is missing.
+
+        :return: A list of strings containing the validated plugin command.
+        """
+        if not self._plugin_command or "--port" in self._plugin_command:
+            return self._plugin_command or []
+
+        return self._plugin_command + ["--port", str(self.__port)]
+
+    def start(self) -> bool:
+        """
+        Starts the backend_plugin process.
+
+        :return: True if the plugin process started successfully, False otherwise.
+        """
+        try:
+            # STDIN needs to be None because we provide no input, but communicate via a local socket instead.
+            # The NUL device sometimes doesn't exist on some computers.
+            self._process = subprocess.Popen(self._validatePluginCommand(), stdin = None)
+            self._is_running = True
+            return True
+        except PermissionError:
+            Logger.log("e", f"Couldn't start backend_plugin [{self._plugin_id}]: No permission to execute process.")
+        except FileNotFoundError:
+            Logger.logException("e", f"Unable to find backend_plugin executable [{self._plugin_id}]")
+        except BlockingIOError:
+            Logger.logException("e", f"Couldn't start backend_plugin [{self._plugin_id}]: Resource is temporarily unavailable")
+        except OSError as e:
+            Logger.logException("e", f"Couldn't start backend_plugin [{self._plugin_id}]: Operating system is blocking it (antivirus?)")
+        return False
+
+    def stop(self) -> bool:
+        if not self._process:
+            self._is_running = False
+            return True  # Nothing to stop
+
+        try:
+            self._process.terminate()
+            return_code = self._process.wait()
+            self._is_running = False
+            Logger.log("d", f"Backend_plugin [{self._plugin_id}] was killed. Received return code {return_code}")
+            return True
+        except PermissionError:
+            Logger.log("e", "Unable to kill running engine. Access is denied.")
+            return False