Add event for LTI 1.1 XBlock grading

[luisfelipec95]

Problem

Currently, Open edX does not emit a dedicated event when an LTI 1.1 XBlock reports a grade back to the platform.

Proposal

Introduce a new event in openedx-events that is emitted when an LTI 1.1 XBlock is graded.

The event should:

• Represent the grading of LTI 1.1 content

• Include the relevant grading context (course, block, learner, grade)

• Follow existing Open edX event and schema conventions

Additional context

A draft implementation has been proposed in: #540

[feanil]

How different is this from https://github.com/openedx/openedx-events/blob/main/openedx_events/learning/signals.py#L502-L507

It seems like that generic signal could be used for LTI as well?