flows: add support for default_context, etc

default_context can be used to influence policies during the planning. This should be used when the Planner is called from other views to correctly preseed the plan.
This also checks if there is a PENDING_USER set, and uses that user for the cache key instead

passbook/sources/oauth/views/core.py

@@ -176,10 +176,14 @@ class OAuthCallback(OAuthClientMixin, View):
         # We run the Flow planner here so we can pass the Pending user in the context
         flow = get_object_or_404(Flow, designation=FlowDesignation.AUTHENTICATION)
         planner = FlowPlanner(flow)
-        plan = planner.plan(self.request)
-        plan.context[PLAN_CONTEXT_PENDING_USER] = user
-        plan.context[PLAN_CONTEXT_AUTHENTICATION_BACKEND] = user.backend
-        plan.context[PLAN_CONTEXT_SSO] = True
+        plan = planner.plan(
+            self.request,
+            {
+                PLAN_CONTEXT_PENDING_USER: user,
+                PLAN_CONTEXT_AUTHENTICATION_BACKEND: user.backend,
+                PLAN_CONTEXT_SSO: True,
+            },
+        )
         self.request.session[SESSION_KEY_PLAN] = plan
         return redirect_with_qs(
             "passbook_flows:flow-executor", self.request.GET, flow_slug=flow.slug,