feat: gotenberg support for PDF block

[Kelketek]

Description

This Pull Request adds support for conversion of documents to PDF via the Gotenberg service if it is available.

Supporting information

• Product Proposal this is implementing
• Frontend Authoring version needed to test this
• Tutor Plugin needed to test this

Testing instructions

1. Follow the instructions in the frontend-app-authoring repository and use the linked version.
2. Install the Tutor plugin for Gotenberg linked above.
3. Clone this repository and switch to this branch. Add it to the mounts for Tutor via tutor mounts add /path/to/xblocks-core
4. tutor images build openedx-dev
5. Conversion to PDF should now be enabled. Go to a course and find PDF in the advanced components. Add it. Select an office document and verify the document is auto-converted and embedded.
6. Upload a PDF to verify that still works fine.
7. Go to a library and do the same-- upload an office document. Verify it's auto-converted and embedded.
8. Do it again with a PDF to verify that still works fine.

[openedx-webhooks]

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