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.  
My argument for this change is that it is inconsistent with almost anything else.  If we consistently used pointers for optional fields, maybe, but we don't.  And pointers add extra logic at every site to check for nil, plus increases the need for fuzzing.  Better to just detect "" and default it, I think.