Added ability for backend plugins to determine their usability

Modified BackendPlugin.py and StartSliceJob.py to allow plugins to decide when they should be used. The 'usePlugin'
method was added to enable a plugin to return a boolean indicating if it should be used or not, e.q.: start up and
connect to the services.

Contributes to CURA-11031

cura/BackendPlugin.py

@@ -22,6 +22,10 @@ class BackendPlugin(AdditionalSettingDefinitionsAppender, PluginObject):
         self._process = None
         self._is_running = False
         self._supported_slots: List[int] = []
+        self._use_plugin = True
+
+    def usePlugin(self) -> bool:
+        return self._use_plugin
 
     def getSupportedSlots(self) -> List[int]:
         return self._supported_slots
@@ -55,6 +59,8 @@ class BackendPlugin(AdditionalSettingDefinitionsAppender, PluginObject):
 
         :return: True if the plugin process started successfully, False otherwise.
         """
+        if not self.usePlugin():
+            return False
         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.

plugins/CuraEngineBackend/StartSliceJob.py

@@ -303,6 +303,8 @@ class StartSliceJob(Job):
             self._buildExtruderMessage(extruder_stack)
 
         for plugin in CuraApplication.getInstance().getBackendPlugins():
+            if not plugin.usePlugin():
+                continue
             for slot in plugin.getSupportedSlots():
                 # Right now we just send the message for every slot that we support. A single plugin can support
                 # multiple slots