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.  
I'm ok either way.  If you leave the attribute out all together it will default to none as well.  Leaving it in in the service config allows you to explicitly state 'None' versus implying it with an empty string.