Support static assets in the Learning Core runtime

[kdmccormick]

@ormsbee Opening this issue now so that I have something to link TODO comments to :)

Exact details TBD. ADR with details: https://github.com/openedx/openedx-learning/blob/main/docs/decisions/0015-serving-static-assets.rst

[kdmccormick]

In review: openedx/openedx-learning#217

[bradenmacdonald]

openedx/frontend-app-authoring#1242 will soon merge, and then we'll pretty much have everything in place on the frontend to be able to test this: e.g. get a simple HTML XBlock that references a static image to render on the frontend. (Would still require using the python API to upload the image and reference it from the block's OLX though.)

I think that is the simplest case, before we move on to things like copying a block into a modulestore course.