Add context to Provider interface

[sugarraysam]

It would be nice if the generated Provider interface would mimic what gRPC does and add a default ctx context.Context as the first argument.

// inside `provider.go`
type Provider interface {
    GetSomething(ctx context.Context, responses *GetSomethingResponses) (err error)
}

// and then in the Handler, simply use the context from the incoming request
// inside `server.go`
func HandleGetSomething(w http.ResponseWriter, r *http.Request) {
    // ... stuff
    err = provider.GetSomething(r.Context(), responses)
    // ... more stuff
}

This would allow passing context from an http.Client, or protecting your handler with a TimeoutMiddleware:

func TimeoutMiddleware(next http.Handler) http.Handler {
	timeout := 7 * time.Second
	msg := `{"error":  "your request timed out"}`
	return http.TimeoutHandler(next, timeout, msg)
}

[bvwells]

Added a PR for this issue. How this is useful!