daemon: allowing adding external http handlers

[paul-rodriguez]

This PR turns the daemon API into an exported variable.

External tools depending on pebble may append to the variable to extend the daemon API, see snippet below.

type fakeHandler struct {
	cmd        *daemon.Command
	lastMethod string
}

func (h *fakeHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	h.lastMethod = r.Method
	fmt.Println("Fake handler called")
}

func main() {

	var handler fakeHandler

	getCallback := func(c *daemon.Command, r *http.Request, s *daemon.UserState) daemon.Response {
		handler.cmd = c
		fmt.Println("getCallback called")
		return &handler
	}

	command := daemon.Command{
		Path:    "/v1/addedendpoint",
		GuestOK: true,
		GET:     getCallback,
	}
	
	// Here's the relevant line:
	daemon.API = append(daemon.API, &command)

	if err := cli.Run(); err != nil {
		fmt.Fprintf(cli.Stderr, "error: %v", err)
		os.Exit(1)
	}
	
	// Executable now handles requests for "/v1/addedendpoint"
}

[benhoyt]

Approved, but note we should change Api -> API

[niemeyer]

Looks good. Just one detail needs to be fixed on the test.

[flotter]

Gustavo reviewed & approved, pending comment fix and conflict resolved

[niemeyer]

Sorry, but still has issues. :(

[niemeyer]

Thanks!