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>
6a19995f13