Gateway: allow linking packages from other namespaces/subdomains

[trieloff]

In OpenWhisk, you can link another user's package into your own namespace and execute it as if it was your own code, but the owner of the original namespace still controls the entire lifecycle of the package's actions. This is a very neat feature that does not have equivalents in other platforms, but would still be desirable to have.

A simple gateway-level implementation could look like this:

1. add another backend to the Fastly service: anywhere.run
2. add an edge dictionary linked_packages where the package name is the key and the subdomain where it is deployed is the value
3. upon request, look up the package and if the package can be found in linked_packages, use the anywhere.run backend for this request, set the target subdomain as the Host header and leave the URL as is
4. When deploying, update the edge dictionary with the specified links