Headless Service mode

[karolz-ms]

We currently have "proxied" Service mode and "proxyless", which is really just "singleton Endpoint with failover" mode.

Some applications/frameworks can do client-side load balancing. For those we should support "headless" service mode. The idea here is still to run without a proxy, but lift the limit on having just a single Endpoint backing the service. Instead, the Endpoint number would be unlimited, and the Service would advertise them via Endpoints status property (containing address/port information). Clients could then watch the (headless) Service object and change their routing accordingly.

This is technically a breaking change to the model, so we can go about it at least two different ways:

1. Introduce a v2 Service object, where EffectiveAddress/EffectivePort is augmented by zero or more Endpoints that are part of Status, supporting all service modes, with all the Kubernetes forward/backward storage shimming.
2. Introduce a different object type, e.g. Service2, still supporting all the service modes, but replacing EffectiveAddress/EffectivePort with Endpoints list.

IMO microsoft/usvc#2 is sufficient and less work, but we probably need more discussion around this.

CC @ReubenBond

[danegsta]

Another option to consider is keeping the existing EffectiveAddress/EffectivePort behavior and adding a new set of AllEffectiveAddresses/AllEffectivePorts properties that would track the full set of addresses/ports (where a given AllEffectiveAddresses index has a corresponding AllEffectivePorts entry at the same index).