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_clone.go

@@ -620,6 +620,7 @@ func (src *UserProfile) Clone() *UserProfile {
 	}
 	dst := new(UserProfile)
 	*dst = *src
+	dst.Groups = append(src.Groups[:0:0], src.Groups...)
 	return dst
 }
 
@@ -629,6 +630,7 @@ var _UserProfileCloneNeedsRegeneration = UserProfile(struct {
 	LoginName     string
 	DisplayName   string
 	ProfilePicURL string
+	Groups        []string
 }{})
 
 // Clone makes a deep copy of VIPService.