Apply suggestions from code review

Co-authored-by: Mira <weller@rami.io>

src/pretix/base/customersso/oidc.py

@@ -140,16 +140,9 @@ def oidc_validate_and_complete_config(config):
             )
 
     if "query_parameters" in config and config["query_parameters"]:
-        for qp in config["query_parameters"].split("&"):
-            # Very rudimentary check to avoid the most common footguns:
-            # - No ? in the query parameters
-            # - Max of one = (to split key and value)
-            # - One key, one value. Not just keys with no value.
-            if not (qp.count('=') == 1 and qp.count('?') == 0 and len(list(filter(None, qp.split('=')))) == 2):
-                raise ValidationError(
-                    _(f'Query parameter {qp} is invalid.')
-                )
-
+         config["query_parameters"] = urllib.parse.urlencode(
+             urllib.parse.parse_qsl(config["query_parameters"])
+         )
     config['provider_config'] = provider_config
     return config
 
@@ -167,8 +160,7 @@ def oidc_authorize_url(provider, state, redirect_uri):
     }
 
     if "query_parameters" in provider.configuration and provider.configuration["query_parameters"]:
-        for qp in provider.configuration["query_parameters"].split("&"):
-            params[qp.split("=")[0]] = qp.split("=")[1]
+        params.update(urllib.parse.parse_qsl(provider.configuration["query_parameters"]))
 
     return endpoint + '?' + urlencode(params)