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: {}