This PR improves how we handle REST API.
Problems before:
* `packages/next/src/routes/rest/*` had a huge amount of code that
didn't depend on next.js at all.
* `packages/next/src/routes/rest/index.ts` itself was not only huge but
also really hard to follow. Every method (`GET`, `POST` etc. was almost
full copy of another).
* `packages/next/src/utilities` had some utilities like
`headersWithCors` or `createPayloadRequest` that again, weren't depend
on next.js and potentially can be used outside of next.js.
Now:
All the logic that's not related to next.js now is inside
`packages/payload`, `packages/next/src/routes/rest/index.ts` now is only
_40_ lines instead of 900+
Functions like `headersWithCors` are now implemented and exported in
`payload`. To keep bc, we re-export them from the same path but marked
as `@deprecated`.
You can attach Payload REST API to any backend framework that uses Fetch
API (like Remix / SolidStart / Bun / Hono) if you don't need the admin
panel in your server instance, but you still want to have REST API. The
main function [`handleEndpoints`
](https://github.com/payloadcms/payload/pull/10466/files#diff-82e97630068f9fc40256f3f46e06226215ab150d16012281810586b51b0cfd51R28)
accepts `Request` and returns `Response`.
It's also doable with Express, but you'd have to convert node.js'
req/res to fetch.
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