tailcfg: reintroduce UserProfile.Groups

This change reintroduces UserProfile.Groups, a slice that contains
the ACL-defined and synced groups that a user is a member of.

The slice will only be non-nil for clients with the node attribute
see-groups, and will only contain groups that the client is allowed
to see as per the app payload of the see-groups node attribute.

For example:
```
"nodeAttrs": [
  {
    "target": ["tag:dev"],
    "app": {
      "tailscale.com/see-groups": [{"groups": ["group:dev"]}]
    }
  },

  [...]

]
```

UserProfile.Groups will also be gated by a feature flag for the time
being.

Updates tailscale/corp#31529

Signed-off-by: Gesa Stupperich <gesa@tailscale.com>

feature/taildrop/ext.go

@@ -139,8 +139,8 @@ func (e *Extension) onChangeProfile(profile ipn.LoginProfileView, _ ipn.PrefsVie
 	e.mu.Lock()
 	defer e.mu.Unlock()
 
-	uid := profile.UserProfile().ID
-	activeLogin := profile.UserProfile().LoginName
+	uid := profile.UserProfile().ID()
+	activeLogin := profile.UserProfile().LoginName()
 
 	if uid == 0 {
 		e.setMgrLocked(nil)