Complete design/implementation of ExecutionStream API

It is currently possible to use the ExecutionStream rpc, using only StartRequest. However, it is somewhat unclear how to implement consumers of the rpc in general, because we don't know what the ExecutionStream is going to look like when it is fully realized. Is UpdateRequest going to become necessary for many use cases?

See TODOs at https://github.com/envoyproxy/nighthawk/blob/master/api/client/service.proto