[PassageMarkdown] Add state for retrieving label information
[PassageMarkdown] Add state for retrieving label information

Summary:
To add passage instructions, we want to know which actual question
label is used in the passage (and sentence label, but this is
probably always `[1]` in practice, while questions will vary
depending on the user's position in the test).

Depends on D17746

Test Plan: Used in the next commit

Reviewers: michelle, emily

Reviewed By: emily

Subscribers: john, #a11y

Differential Revision: https://phabricator.khanacademy.org/D17747

daukantas/perseus,ariabuckles/perseus,iamchenxin/perseus,alexristich/perseus,ariabuckles/perseus,alexristich/perseus,chirilo/perseus,abe4mvp/perseus,iamchenxin/perseus,ariabuckles/perseus,learningequality/perseus,chirilo/perseus,learningequality/perseus,sachgits/perseus,daukantas/perseus,sachgits/perseus,abe4mvp/perseus,ariabuckles/perseus