fix: awesome private body registration

[Stef-Rousset]

🎩 Description

This PR update the create_proposal extends to integrate the registration of the private_body from awesome.

Testing

1. As an admin, in the BO, go to decidim awesome tab > Proposals custom fields > Private fields, add some fields (screenshot one)
2. As an admin, go to the proposal's process where you added the private fields, create a new proposal from the BO. Ensure it is well saved. Click en "Answer proposal" button and check that the private body is filled (screenshot two).
3. As a user in the FO, create a new proposal and publish it.
4. As an admin repeat step 2 for this proposal
5. As an admin, export the proposals and ensure the private fields are well filled.

📌 Related Issues

Link your PR to an issue

• Fixes https://github.com/orgs/OpenSourcePolitics/projects/26/views/1?pane=issue&itemId=158789385&issue=OpenSourcePolitics%7Cintern-tasks%7C376

Tasks

• Add specs

📷 Screenshots

ONE

TWO

THREE