feat!: modulestore_migrator (#36873)

This introduces the modulestore_migrator app, which can be
used to copy content (courses and libraries) from modulestore
into Learning Core. It is currently aimed to work on the legacy
library -> v2 library migration, but it will be used in the future
for course->library and course->course migrations.

This includes an initial REST API, Django admin interface,
and Python API.

Closes: https://github.com/openedx/edx-platform/issues/37211

Requires some follow-up work before this is production-ready:
https://github.com/openedx/edx-platform/issues/37259

Co-authored-by: Andrii <andrii.hantkovskyi@raccoongang.com>
Co-authored-by: Maksim Sokolskiy <maksim.sokolskiy@raccoongang.com>

.github/workflows/unit-test-shards.json

@@ -238,6 +238,7 @@
       "cms/djangoapps/cms_user_tasks/",
       "cms/djangoapps/course_creators/",
       "cms/djangoapps/export_course_metadata/",
+      "cms/djangoapps/modulestore_migrator/",
       "cms/djangoapps/maintenance/",
       "cms/djangoapps/models/",
       "cms/djangoapps/pipeline_js/",