runtimes/core: support running multiple gateways

[fredr]

Refactor gateway in runtime so we can support multiple gateways. This is a first step of several to get multiple gateways working.

It adds a internal route to each gateway, which is a all routes with the service names as prefixes (but requires platform signed requests). The internal route will be used if the header x-encore-internal-route is set.

[encore-cla[bot]]

All committers have signed the CLA.