feat: support fetch for Next.js app-router

[stipsan]

Testable using:

npm i @sanity/client@fetch --save-exact

It allows opt-in to using native fetch in the nodejs runtime in Next.js:

import {createClient} from '@sanity/client'

const client = createClient({
  ...config,
  // use `fetch`, opts-in to automatic fetch deduping and other benefits in app-router
  fetch: true,
})

And in both nodejs, and now edge, you can set certain useful options like cache and next on the request level:

const data = await client.fetch(query, params, {
  cache: 'force-cache',
  next: {revalidate: 30, tags: ['page']}
})

This allows us to get rid of workarounds such as: https://github.com/sanity-io/next-sanity#app-router-react-server-components-and-caching
And the ability to use features that are only available to data fetching that use fetch
under the hood.

[gksander]

Awesome! Tried this on my end in a production app running Next.js 13 (using fetch cache tags etc etc), and it appears to Just Work ™️

[indeox]

Tested this as well, and works beautifully. Very much looking forward to when it gets merged! 🎉 It will solve so many issues