Send request to specific region

Currently, a request to a Worker (pending name #105) is automatically routed to the instance in the best region which is usually the closest. It might be useful to be able to specify an instance in a specific region in the request [^cdnplanet] [^cf]. Note, this is different from deploying the Worker only to specific regions in the first place, see #93.

One option could be to use the URL. To not interfere with the API of the Worker which can access the path, a subdomain could be used, e.g. `..deno.dev`. If no `` subdomain is given it uses the default routing like right now.

But if in the future more settings are added this would prove cumbersome since in a URL the order of subdomains is fixed and for any subdomain all previous subdomains have to be set, e.g. `..`. Likely there aren't many more settings for Workers, but there is already one for Coordinators (#88) which uses a subdomain for a duplicate Coordinator.

A better option could be to use a custom HTTP header, e.g. `X-Deno-Deploy: region=`. The header would be filtered out at the Deno Deploy edge and not be visible to the Worker that receives the request. The advantage of key-value pairs in a header is that the order is not fixed and any one can be left out. The advantage is similar to using a single object as function argument instead of named arguments.

[^cdnplanet]: https://www.cdnplanet.com/blog/our-cloudflare-workers-kv-wishlist/#3
[^cf]: https://community.cloudflare.com/t/is-it-possible-to-trigger-a-worker-on-a-specific-geographic-region/25175

Start by reviewing the proposed URL and HTTP-header approaches, along with related issues #105, #93, and #88. Done means a request can select a specific region, requests without that selection retain default routing, and the chosen mechanism does not expose routing settings to the Worker.

Written by the indexing model from the issue text.