Queueing and mapping utilities for outbound data sync (#4881)

Add a registry for datasync providers and an associated sync queue, to be used by 
plugins that transfer data from pretix orders to external systems. 
Additionally, provide a generic data mapping interface to be used in settings pages 
of such plugins, to let users configure which information from pretix to fill into
which data fields of the external system.

---------

Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/base/signals.py

@@ -257,8 +257,14 @@ class Registry:
                      When a new entry is registered, all accessor functions are called with the new entry as parameter.
                      Their return value is stored as the metadata value for that key.
         """
-        self.registered_entries = dict()
         self.keys = keys
+        self.clear()
+
+    def clear(self):
+        """
+        Removes all entries from the registry.
+        """
+        self.registered_entries = dict()
         self.by_key = {key: {} for key in self.keys.keys()}
 
     def register(self, *objs):
@@ -333,6 +339,23 @@ class EventPluginRegistry(Registry):
     def __init__(self, keys):
         super().__init__({"plugin": lambda o: get_defining_app(o), **keys})
 
+    def filter(self, active_in=None, **kwargs):
+        result = super().filter(**kwargs)
+        if active_in is not None:
+            result = (
+                (entry, meta)
+                for entry, meta in result
+                if is_app_active(active_in, meta['plugin'])
+            )
+        return result
+
+    def get(self, active_in=None, **kwargs):
+        item, meta = super().get(**kwargs)
+        if meta and active_in is not None:
+            if not is_app_active(active_in, meta['plugin']):
+                return None, None
+        return item, meta
+
 
 event_live_issues = EventPluginSignal()
 """