feat: add and switch to downstream-only fields [FC-0076]

[pomegranited]

Description

Adds the concept of "downstream-only" fields to the XBlock upstream sync logic.

Downstream-only fields are customizable fields which are set only on the downstream XBlock -- we don't keep track of the upstream field value anywhere on the downstream XBlock. Changes made to these fields in the upstream block are ignored, and if the link to the upstream block is severed, the downstream changes are preserved (not reset back to defaults, like the upstream-tracked customizable fields are).

The list of fields chosen as "downstream-only" are fields related to scoring and grading.

The max_attempts field was previously a customizable field where the upstream value was tracked, but because it is a scoring-related field, it's been converted to a "downstream-only" field.

This change impacts Course Authors using Library content in their courses.

Supporting information

Part of: openedx/frontend-app-authoring#1317
Relates to: openedx/frontend-app-authoring#1601
Private-ref: FAL-3996

Testing instructions

Backend changes are covered by unit tests.

For manual testing instructions, see openedx/frontend-app-authoring#1601

Deadline

None

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[robrap]

Passing comment:
Updating the title to something like the following would highlight more of the PR description:

feat: add and switch to downstream-only fields

[navinkarkera]

@pomegranited Looks good. 👍

• I tested this: (Verified syncing upstream does not affect downstream only fields)
• I read through the code
• I checked for accessibility issues
• Includes documentation

[ChrisChV]

Looks good! I will merge this on monday

[kdmccormick]

Thank you for integrating this in so seamlessly with the existing upstream sync concept!