add SessionAffinity type to ServiceSpec

This PR is the first step for enhancing the load balancer in the service proxy to support sticky sessions - Issue #2867.  This type will be used in enabled through the service config json/yaml.  
personally, I just let the tests fail ;)

But makes sense to me...

Seems like people have had their chance to peek @ the API, can you add it to the other types, and we'll merge this?

Thanks!
--brendan