Make setting "Grass7AlgorithProvider.activateSetting" optional.

This change makes it easier to subclass `Grass7AlgorithProvider` which
allows e.g. to expose GRASS Addons as QGIS plugins.

Disclaimer: `Grass7AlgorithProvider` is a private API and it may change
            without warning. 3rd parties that rely on this should expect
            that their code will break in future releases.
            For more info please check the comments at #30252

Nevertheless, it currently is possible to subclass with something like this:

    class CustomGrassBasedProvider(Grass7AlgorithmProvider):
        # Set descriptionFolder to our own description directory
        descriptionFolder = os.path.join(os.path.dirname(__file__), 'description')

        # 3rd party plugins don't need an activation/deactivation setting
        activateSetting = None

        # define the rest of the methods that are needed (e.g. name(), id() etc)
        # ...

Fixes #30241
Continues #9202

Author: pmav99

python/plugins/processing/algs/grass7/Grass7AlgorithmProvider.py

@@ -40,10 +40,7 @@
 
 class Grass7AlgorithmProvider(QgsProcessingProvider):
 
-    # Subclasses of `Grass7AlgorithmProvider` should override `descriptionFolder`
-    # and set its value to their own description folder.
     descriptionFolder = Grass7Utils.grassDescriptionPath()
-
     activateSetting = "ACTIVATE_GRASS7"
 
     def __init__(self):
@@ -52,8 +49,9 @@ def __init__(self):
 
     def load(self):
         ProcessingConfig.settingIcons[self.name()] = self.icon()
-        ProcessingConfig.addSetting(Setting(self.name(), self.activateSetting,
-                                            self.tr('Activate'), True))
+        if self.activateSetting:
+            ProcessingConfig.addSetting(Setting(self.name(), self.activateSetting,
+                                                self.tr('Activate'), True))
         if isMac():
             ProcessingConfig.addSetting(Setting(
                 self.name(),
@@ -85,7 +83,8 @@ def load(self):
         return True
 
     def unload(self):
-        ProcessingConfig.removeSetting(self.activateSetting)
+        if self.activateSetting:
+            ProcessingConfig.removeSetting(self.activateSetting)
         if isMac():
             ProcessingConfig.removeSetting(Grass7Utils.GRASS_FOLDER)
         ProcessingConfig.removeSetting(Grass7Utils.GRASS_LOG_COMMANDS)
@@ -94,10 +93,13 @@ def unload(self):
         ProcessingConfig.removeSetting(Grass7Utils.GRASS_USE_VEXTERNAL)
 
     def isActive(self):
-        return ProcessingConfig.getSetting(self.activateSetting)
+        if self.activateSetting:
+            return ProcessingConfig.getSetting(self.activateSetting)
+        return True
 
     def setActive(self, active):
-        ProcessingConfig.setSettingValue(self.activateSetting, active)
+        if self.activateSetting:
+            ProcessingConfig.setSettingValue(self.activateSetting, active)
 
     def createAlgsList(self):
         algs = []