feature,client: add serviceclientprefs for desktop client service launch (#20501)

Add serviceclientprefs, an optional feature that stores and loads the
desktop clients' saved service launch preferences, one file per login
profile.

- Add GET|POST /localapi/v0/prefs/service-clients to load and save the
  current profile's service client prefs.
- Add local client GetServiceClientPrefs and SetServiceClientPref that
  call the new local api endpoint.
- Store the prefs with the ipn/store FileStore at
  TailscaleVarRoot()/profile-data/<profileID>/service-client-prefs/<hex-encoded-key>,
  so DeleteProfile cleans them up for free. Fall back to an in-memory
  store when there's no var root.
- Register the feature and its local api route from build tagged files
  so the whole thing drops out under ts_omit_serviceclientprefs.
- Add the serviceclient package holding Pref and Prefs (saved client,
  username, database name, and last used time), so the local api client
  and desktop apps can import the types without the feature machinery.

Change-Id: I340a99c1b332d181fb1556fbf3e8003bb3b95a08
Updates: https://github.com/tailscale/tailscale/issues/20429

Signed-off-by: Rollie Ma <rollie@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -322,6 +322,8 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/feature/routecheck                             from tailscale.com/feature/condregister
         tailscale.com/feature/runtimemetrics                         from tailscale.com/feature/condregister
    L    tailscale.com/feature/sdnotify                               from tailscale.com/feature/condregister
+        tailscale.com/feature/serviceclientprefs                     from tailscale.com/feature/condregister
+        tailscale.com/feature/serviceclientprefs/serviceclient       from tailscale.com/client/local+
   LD    tailscale.com/feature/ssh                                    from tailscale.com/cmd/tailscaled
    L    tailscale.com/feature/syslog                                 from tailscale.com/feature/condregister
         tailscale.com/feature/syspolicy                              from tailscale.com/feature/condregister

cmd/tailscaled/deps_test.go

@@ -13,6 +13,18 @@ import (
 	"tailscale.com/tstest/deptest"
 )
 
+func TestOmitServiceClientPrefs(t *testing.T) {
+	const msg = "unexpected with ts_omit_serviceclientprefs"
+	deptest.DepChecker{
+		GOOS:   "linux",
+		GOARCH: "amd64",
+		Tags:   "ts_omit_serviceclientprefs,ts_include_cli",
+		BadDeps: map[string]string{
+			"tailscale.com/feature/serviceclientprefs": msg,
+		},
+	}.Check(t)
+}
+
 func TestOmitSSH(t *testing.T) {
 	const msg = "unexpected with ts_omit_ssh"
 	deptest.DepChecker{