feat(server): automatically extract forwarded-for header

[deer]

closes #1827
Or rather, the discussion that happens later in the issue.

This change modifies the call to Deno.serve to automatically extract the X-Forwarded-For header, if it exists. This is then available in HandlerContext, MiddlewareHandlerContext, and RouteContext.

[deer]

@marvinhagemeister, based on #1827 (comment) I guess I should close this. Before I do though: do you see any value in having the handler passed to Deno.serve configurable/overridable? If not, I'll close it. If yes, I can rework this.

Right now I don't see any way of controlling middleware execution order, so if someone absolutely needs something to run first, they're out of luck. Being able to override the Deno.serve handler and having the ability to order middleware (especially from plugins) seems like it would increase flexibility. I can open a separate issue for middleware ordering if desired.