xds: fix VHDS empty subscription behavior to prevent wildcarding subscription

[whutwhu]

Description
This PR fixes an issue where the control plane incorrectly treated an empty resource_names_subscribe list in a VHDS DeltaDiscoveryRequest as a wildcard subscription.

According to the Virtual Host Discovery Service (VHDS) documentation, VHDS is designed for on-demand loading. Interpreting an empty subscription list as a wildcard is counter-productive to the "lazy-loading" goal of VHDS, as it could cause the control plane to push all virtual hosts to the proxy unexpectedly.

Changes

1. Added logic to the xDS server to detect if the resource type is VHDS.
2. If the resource type is VHDS and the resource_names_subscribe list is empty, the server will now treat the subscription as empty rather than fetching all resources.
3. Preserved existing wildcard behavior for Listener, Cluster, Route, and Endpoint types.

Risk Level
Low. This specifically targets VHDS resource types and aligns with the expected xDS protocol behavior for on-demand resources.

Testing Done
I performed manual validation via the Observer to ensure the subscription is ignored when the resource is VHDS with either a wildcard or an empty subscription.

[gu0keno0]

Can you also add a test plan?

[gu0keno0]

Nit: use const https://github.com/envoyproxy/go-control-plane/blob/164e01cbb8996d13ae45228bd0a53e575a01f038/pkg/resource/v3/resource.go#L22

[whutwhu]

Sounds good!

[whutwhu]

Can you also add a test plan?

I’ve added some unit tests and also did manual validation via the Observer to ensure the subscription is ignored when the resource is VHDS with either a wildcard or an empty subscription.