feat: implement darwin launch agent (#192)

* feat: sketch out periodic command

* feat: sketch out periodic command for macOS

* feat: implement darwin's launch agent

* refactor: better way to run on darwin

Make sure we have code that builds on all platforms.

* fix(run): max 10 URLs with darwin in unattended mode

* feat: add support for seeing/streaming logs

* feat: implement the status command and add usage hints

* feat(periodic): run onboarding if needed

* fix: no too confusing function names

* fix: s/periodic/autorun/

Discussed earlier this morning with @hellais.

* fix: we cannot show logs before Big Sur

Bug reported by @hellais.

internal/cli/run/run.go

@@ -1,6 +1,8 @@
 package run
 
 import (
+	"runtime"
+
 	"github.com/alecthomas/kingpin"
 	"github.com/apex/log"
 	"github.com/fatih/color"
@@ -74,6 +76,13 @@ func init() {
 
 	unattendedCmd := cmd.Command("unattended", "")
 	unattendedCmd.Action(func(_ *kingpin.ParseContext) error {
+		if runtime.GOOS == "darwin" {
+			// Until we have enabled the check-in API we're called every
+			// hour on darwin and we need to self throttle.
+			// TODO(bassosimone): switch to check-in and remove this hack.
+			const veryFew = 10
+			probe.Config().Nettests.WebsitesURLLimit = veryFew
+		}
 		return functionalRun(func(name string, gr nettests.Group) bool {
 			return gr.UnattendedOK == true
 		})