Add sessionServer.labels for custom session server Service labels

What does this MR do?

Adds a sessionServer.labels value to the Helm chart, mirroring the existing service.labels support. When set, the labels are merged into the metadata of the Service template (templates/service-session-server.yaml), alongside the chart's default labels.

Why was this MR needed?

Other Services created by this chart (e.g. the metrics service) already support custom labels via a dedicated labels value, but the session server Service only exposed annotations. In our environment, we need to add a label to the Session Server Service for an internal tool.