feat: allow overriding unit icons (#21433)

This:
1. Introduces a new override using the `pluggable_override` decorator.
It is now possible to specify a custom way of getting XBlock's icon
by defining `GET_UNIT_ICON_IMPL` in settings.
2. Introduces a way to add custom `XBLOCK_MIXINS` by defining
`XBLOCK_EXTRA_MIXINS` in settings. This allows, e.g. to add
new fields to XBlocks.

lms/devstack.yml

@@ -563,6 +563,7 @@ VIDEO_UPLOAD_PIPELINE:
     ROOT_PATH: ''
 WIKI_ENABLED: true
 WRITABLE_GRADEBOOK_URL: null
+XBLOCK_EXTRA_MIXINS: []
 XBLOCK_FS_STORAGE_BUCKET: null
 XBLOCK_FS_STORAGE_PREFIX: null
 XBLOCK_SETTINGS: {}

lms/envs/common.py

@@ -1424,6 +1424,7 @@ from xmodule.x_module import XModuleMixin
 # This should be moved into an XBlock Runtime/Application object
 # once the responsibility of XBlock creation is moved out of modulestore - cpennington
 XBLOCK_MIXINS = (LmsBlockMixin, InheritanceMixin, XModuleMixin, EditInfoMixin)
+XBLOCK_EXTRA_MIXINS = ()
 
 # .. setting_name: XBLOCK_SELECT_FUNCTION
 # .. setting_default: prefer_xmodules

lms/envs/production.py

@@ -1047,3 +1047,6 @@ EXPLICIT_QUEUES = {
 }
 
 LOGO_IMAGE_EXTRA_TEXT = ENV_TOKENS.get('LOGO_IMAGE_EXTRA_TEXT', '')
+
+############## XBlock extra mixins ############################
+XBLOCK_MIXINS += tuple(XBLOCK_EXTRA_MIXINS)