Use request host in SelectCluster

[EngHabu]

Why are the changes needed?

SelectCluster previously returned a statically configured dataplaneDomain (the dataproxy baseURL) as the cluster endpoint for every caller. That forced all clients to talk to the same hostname regardless of how they actually reached the service — breaking setups where the same deployment is fronted by multiple hostnames (e.g. per-tenant/per-region ingresses, local port-forwards, or internal vs. external URLs).

Using the request's Host header lets the server echo back whatever host the client used to reach it, so clients keep talking to the endpoint they already have a working route to.

What changes were proposed in this pull request?

• SelectCluster now reads the Host header off the inbound request and returns it as ClusterEndpoint.
• Removed the dataplaneDomain field and constructor argument from ClusterService; NewClusterService() no longer needs the configured base URL.
• Updated dataproxy/setup.go to call the simplified constructor, plus a minor import ordering cleanup.

How was this patch tested?

Manual verification via the dataproxy endpoint — SelectCluster now returns the host the client used to reach it rather than the configured dataplane domain.

Labels

• fixed

Check all the applicable boxes

• All commits are signed-off.