Add repo purge callbacks and register TUF metadata deletion as one

data/model/repository.py

@@ -105,6 +105,10 @@ def purge_repository(namespace_name, repository_name):
     return False
 
   fetched.delete_instance(recursive=True, delete_nullable=False)
+  
+  # Run callbacks
+  for callback in config.repo_cleanup_callbacks:
+    callback(namespace_name, repository_name)
 
   return True