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

[guillett]

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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:

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

[guillett]

I was hoping that opening draft PR would trigger CI. Apparently it did not.

[guillett]

Superseeded by #7188.