[ppr] Configuration for max react headers length (#67715)
### What?
This exposes a new `reactMaxHeadersLength` config option for
`next.config.js`:
```js
module.exports = {
reactMaxHeadersLength: 1000,
}
```
That allows configuration of the maximum value of all the headers
generated from React. Currently only added when the experimental partial
prerendering (PPR) feature is enabled, this today only affects the size
of the `Link` header that is emitted which contains preloads for assets
like fonts.
This also bumps the default value from 600 to 6000.
### Why?
Some proxies may have differing support for max header sizes, so this
was added as a configuration option to allow users to better control the
output experience in these cases. At the time of writing, most proxies
support a maximum of 8k bytes in the header, which is above the default
6k, allowing for other headers written by Next.js.
---------
Co-authored-by: Sam Ko <sam@vercel.com>
