[FLINK-26340][statefun-golang-sdk] Add ability in Golang SDK to create new statefun.Context from existing one, but with a new underlying context.Context Change sync.Mutex in statefunContext to be *sync.Mutex, to make it copyable, and update construction of statefunContext accordingly Add WithContext to statefun.Context interface and implement on statefunContext Add unit test This closes #303