Simplify interstitial fetching

[nikosdouvlis]

Type of change

• [ ] 🐛 Bug fix
• [ ] 🌟 New feature
• [ ] 🔨 Breaking change
• [ ] 📖 Refactoring / dependency upgrade / documentation
• [ ] other:

Packages affected

• [ ] @clerk/clerk-js
• [ ] @clerk/clerk-react
• [x] @clerk/nextjs
• [x] @clerk/remix
• [ ] @clerk/types
• [ ] @clerk/clerk-expo
• [x] @clerk/backend-core
• [x] @clerk/clerk-sdk-node
• [x] @clerk/edge
• [ ] build/tooling/chore

Description

• [x] npm test runs as expected.
• [x] npm run build runs as expected.

Small refactor to move the responsibility of fetching the interstitial page to each wrapping package (nextjs, remix, gatsby). Can be also considered as preparatory work so it'll be more straightforward to remove the interstitial BAPI call in the near future.

Extracted from https://github.com/clerkinc/javascript/pull/197

[igneel64]

Me and @nikosdouvlis are probably going to be checking today if we can change this logic to plain string from backend-core, removing the need to fetch.

[nikosdouvlis]

Closing this one. We're going to revisit this refactoring if we need to when we handle the new Nextjs 12 middleware structure cc @SokratisVidros