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.

docs/guides/extension_points.rst

@@ -134,6 +134,9 @@ Here are the different integration points that python plugins can use:
    * - XBlock unit tests (``xblock.test.v0``)
      - Assess, Limited
      - XBlocks can also install test code that will then be run alongside the platform's usual python unit tests. It's unclear how well-supported this is at the moment.
+   * - Pluggable override (``edx_django_utils.plugins.pluggable_override.pluggable_override``)
+     - Trial, Stable
+     - This decorator allows overriding any function or method by pointing to an alternative implementation in settings. Read the |pluggable_override docstring|_ to learn more.
 
 .. _Application: https://docs.djangoproject.com/en/3.0/ref/applications/
 .. _Django app plugin documentation: https://github.com/edx/edx-platform/blob/master/openedx/core/djangoapps/plugins/README.rst
@@ -146,6 +149,8 @@ Here are the different integration points that python plugins can use:
 .. _learning_context.py: https://github.com/edx/edx-platform/blob/master/openedx/core/djangoapps/xblock/learning_context/learning_context.py
 .. |UserPartition docstring| replace:: ``UserPartition`` docstring
 .. _UserPartition docstring: https://github.com/edx/edx-platform/blob/f8cc58618a39c9f7b8e9e1001eb2d7a10395797e/common/lib/xmodule/xmodule/partitions/partitions.py#L105-L120
+.. |pluggable_override docstring| replace:: ``pluggable_override`` docstring
+.. _pluggable_override docstring: https://github.com/edx/edx-django-utils/blob/master/edx_django_utils/plugins/pluggable_override.py
 
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