feat: pdf editor authoring flag

[Kelketek]

Description

This merge request adds a waffle flag for toggling the new PDF editor in the authoring MFE, and also signals to the authoring frontend to use the new interface as applicable.

Supporting information

This is part of https://openedx.atlassian.net/wiki/spaces/OEPM/pages/5335908397/Proposal+Add+PDF+Block+to+Base+Installation

Testing instructions

1. Set up this branch of xblocks-contrib to get an MFE editing-compatible version of the PDF Block
2. Set up this branch of frontend-authoring to get the PDF editing tool in the MFE
3. Add "pdf" to your list of advanced modules for a course in studio
4. Add a PDF block and customize it.
5. Save the block
6. Use the edit button to edit the block settings.
7. Add the waffle flag legacy_studio.pdf_editor and set it to enabled globally.
8. Refresh the unit authoring page and hit the edit button on the XBlock. The old authoring interface should appear.

Dependencies

Must be merged BEFORE THIS PR: openedx/frontend-app-authoring#2916
Must be merged BEFORE THIS PR: openedx/xblocks-contrib#193

Deadline

Must be merged in time for Verawood

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