feat(standalone): allow configuring KEEP_ALIVE_TIMEOUT via env var (#46052)
Resolves #39689, partially resolves #28642 (see notes below)
Inspired by #44627
In #28642 it was also asked to expose `server.headersTimeout`, but it is
probably not needed for most use cases and not implemented even in `next
start`. It was needed to change this option before
https://github.com/nodejs/node/issues/27363.
There also exists a rare bug that is described here
https://github.com/nodejs/node/pull/32329#issuecomment-614119125. To fix
this exposing `server.headersTimeout` might be required both in
`server.js` and in `next start`.
Co-authored-by: JJ Kasper <jj@jjsweb.site>