feat: Add Xblock completion mode to course sink

[bmtcril]

In order to know which Xblocks have the ability to be completed before any student actually triggers a completion on them, we would like to have the "completion mode" for the blocks sent to the course sink: https://github.com/openedx/XBlock/blob/3304375ee47d428db769578bd00c079a6c75d015/xblock/completable.py#L10

This can go in the JSON blob and be parsed out downstream, but will allow us to better display completion rates early in the course.

[mavidser]

Hi @bmtcril. Does #49 close this?

[Ian2012]

@mavidser yes, it does