Move preview props into separate manifest (#96004)
Store/load the encryption keys at `.next/server/preview-props.json`
(which are always needed) as opposed to including the giant
`.next/prerender-manifest.json`
The manifest is added to `required-server-file.json#files` so it keeps
getting included in prod
`.next/prerender-manifest.json` is unchanged for backwards compatiblity
reasons
This is work towards the goal of not including the one giant
prerender-manifest at runtime in the serverless function
