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 had to add the fuzz function because I only updated the api/types.go file with the sessionAffinity attribute.  Without the fuzz the test would fail when comparing the versions.  Brendan had suggest I only update the api/types.go file in this PR and once approved I can update the v1beta1 and v1beta2 versions of types.go - at that point I can remove the fuzz.  Sorry for not including this detail in my commit message.  Let me know if there is a better way to achieve this.