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>

ipn/ipnlocal/local.go

@@ -6296,6 +6296,9 @@ func (b *LocalBackend) ProfileMkdirAll(id ipn.ProfileID, subs ...string) (string
 	return b.profileMkdirAllLocked(id, subs...)
 }
 
+// ProfileDataDir is the directory name, under the Tailscale var root, that holds per-profile data.
+const ProfileDataDir = "profile-data"
+
 // profileDataPathLocked returns a path of a profile-specific (sub)directory
 // inside the writable storage area for the given profile ID. It does not
 // create or verify the existence of the path in the filesystem.
@@ -6310,7 +6313,7 @@ func (b *LocalBackend) profileDataPathLocked(id ipn.ProfileID, subs ...string) s
 	if vr == "" {
 		return ""
 	}
-	return filepath.Join(append([]string{vr, "profile-data", string(id)}, subs...)...)
+	return filepath.Join(append([]string{vr, ProfileDataDir, string(id)}, subs...)...)
 }
 
 // profileMkdirAllLocked implements ProfileMkdirAll.