Add configuration for extra middleware classes

CHANGELOG.md

+- Role: edxapp
+  - Added `EDXAPP_EXTRA_MIDDLEWARE_CLASSES` for configuring additional middleware logic.
+
 - Role: discovery
   - Added `OPENEXCHANGERATES_API_KEY` for retrieving currency exchange rates.
 

playbooks/roles/edxapp/defaults/main.yml

@@ -434,6 +434,11 @@ EDXAPP_INSTALL_PRIVATE_REQUIREMENTS: false
 #   - name: git+https://git.myproject.org/MyProject#egg=MyProject
 EDXAPP_EXTRA_REQUIREMENTS: []
 
+# List of custom middlewares that should be used in edxapp to process
+# incoming HTTP resquests. Should be a list of plain strings that fully
+# qualify Python classes or functions that can be used as Django middleware.
+EDXAPP_EXTRA_MIDDLEWARE_CLASSES: []
+
 EDXAPP_GOOGLE_ANALYTICS_ACCOUNT: "None"
 
 EDXAPP_OPTIMIZELY_PROJECT_ID: "None"
@@ -1135,6 +1140,7 @@ generic_env_config:  &edxapp_generic_env
 
   POLICY_CHANGE_GRADES_ROUTING_KEY: "{{ EDXAPP_POLICY_CHANGE_GRADES_ROUTING_KEY }}"
   PROCTORING_SETTINGS: "{{ EDXAPP_PROCTORING_SETTINGS }}"
+  EXTRA_MIDDLEWARE_CLASSES: "{{ EDXAPP_EXTRA_MIDDLEWARE_CLASSES }}"
 
 lms_auth_config:
   <<: *edxapp_generic_auth