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.  
If you remove "Enabled" as suggested above, I think you can collapse this to

```go
SessionAffinity *AffinityType `json:"sessionAffinity,omitempty"`
```