Add a proxy to have fast read access on git-based backends

[guillett]

Superseeds #7184

Summary This is an attempt to have a functional DecapCMS instance with more than +500 entries and relationship between entities.

Indeed, we (https://contribuer-aides-jeunes.netlify.app/) currently rely on a github gateway backend but, we suffer from poor loading performance and api rate limiting issue on github API.

Test plan

I plan on adding one. It mainly relies on existing code.

Checklist

Please add a x inside each checkbox:

• [x] I have read the contribution guidelines.
• [x] I have ran npm run type-check
• [ ] I added tests
• [ ] I have ran npm run test
• [ ] I have ran npm run format