[BB-6206] Adds a openedx-filter event to hook into XBlock Rendering

[tecoholic]

Description

This PR introduces a new openedx-filters event org.openedx.learning.xblock.render.started.v1 hook in the platform. This is implemented a part of the PoC to implement "Edit Link" in the course content for Open Source courses.

Users impacted by the change:

• "Students"

Supporting information

• Discussion about the proposal for "Edit Links"

Testing instructions

This PR is 1 of the 3 PRs that implement the Edit Link PoC and should be tested in tandem.

• Follow instructions in the openedx-edit-links PR to test these changes.

Other related PRs

1. openedx-filters - [BB-6206] Introduce a Filter to hook into courseware XBlock rendering openedx-filters#35
2. openedx-edit-links - Implement "Edit on Git" link for each section open-craft/openedx-edit-links#1

Deadline

"None" if there's no rush, or provide a specific date or event (and reason) if there is one.

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[bradenmacdonald]

There are many places that XBlocks can be rendered - in LMS courseware, in Studio when editing courses, in LMS Libraries via direct learning (LabXchange is the only example), or when editing Libraries in Studio.

Is the intention of this PR that it is strictly for XBlocks being rendered for courses in the LMS, or for any time an XBlock is rendered in any learning context? I would say the latter is the most flexible. For example, then you could use it to add authoring tools in Studio too.

Whichever it is, the documentation should be very explicit about that, but right now I think it's ambiguous.

As another example, the docstring currently says that this is only used with units (vertical blocks), but the render_xblock method that you've added it to here can be used to render any kind of XBlock. For example, with the mobile app, it often renders a single XBlock directly, such as a problem block.

It's also worth noting that in the mobile app, XBlocks which have native UI are only downloaded using their data via student_view_data so this won't affect them. It only affects XBlocks that are rendered in a webview.

[tecoholic]

@bradenmacdonald Thank you for providing a great context. Frankly, I didn't think of the various contexts in which the XBlock could be rendered and you are absolutely right about the description being vauge. Let me dig into this more and find the right place to place this hook and also document the context better.

The intention initially was to place a modify the content per unit (hence the target on VerticalBlocks). Based on the testing and the followup discussion, it is becoming clear that this should be targeted at individual XBlock level.

I moving this to draft while the ambiguity is resolved.

[natabene]

@tecoholic Thank you for your contribution. Please let me know once this is ready for our review.

[tecoholic]

Closing this in favor of #30773

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