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>

tailcfg/tailcfg_view.go

@@ -2505,8 +2505,15 @@ func (v UserProfileView) ID() UserID { return v.ж.ID }
 func (v UserProfileView) LoginName() string { return v.ж.LoginName }
 
 // "Alice Smith"
-func (v UserProfileView) DisplayName() string           { return v.ж.DisplayName }
-func (v UserProfileView) ProfilePicURL() string         { return v.ж.ProfilePicURL }
+func (v UserProfileView) DisplayName() string   { return v.ж.DisplayName }
+func (v UserProfileView) ProfilePicURL() string { return v.ж.ProfilePicURL }
+
+// Groups is a subset of SCIM groups (e.g. "engineering@example.com")
+// or group names in the tailnet policy document (e.g. "group:eng")
+// that contain this user and that the coordination server was
+// configured to report to this node.
+// The list is always sorted when loaded from storage.
+func (v UserProfileView) Groups() views.Slice[string]   { return views.SliceOf(v.ж.Groups) }
 func (v UserProfileView) Equal(v2 UserProfileView) bool { return v.ж.Equal(v2.ж) }
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
@@ -2515,6 +2522,7 @@ var _UserProfileViewNeedsRegeneration = UserProfile(struct {
 	LoginName     string
 	DisplayName   string
 	ProfilePicURL string
+	Groups        []string
 }{})
 
 // View returns a read-only view of VIPService.