How to handle host headers

[vicb]

ref/discussion: see #752

When OpenNext is behind a reverse proxy (i.e. aws wrappers) the host can be retrieved from header["x-forwarded-host"].
For Node, it depends how the infra is setup (behind a RP or not).

We also have this:

opennextjs-aws/packages/open-next/src/core/requestHandler.ts

Lines 46 to 48 in e48951f

	if (initialHeaders["x-forwarded-host"]) {
	initialHeaders.host = initialHeaders["x-forwarded-host"];
	}

header["x-forwarded-host"] should not be trusted if not behind a reverse proxy as it can be forged.

We should figure out the best way to configure this.