No way to configure headless service independently from the main service

Problem

The headless service (qdrant-headless) is fully derived from service.* values — there are no dedicated headlessService.* values to override its configuration independently. There is also no way to disable it entirely.

Currently service-headless.yaml reuses service.annotations, service.additionalLabels, and service.ports without any way to override or suppress them for the headless service specifically.

The headless service is only useful in cluster mode (config.cluster.enabled: true). When running a single-node Qdrant instance, it serves no purpose but still gets created unconditionally.

Expected behavior

Add a headlessService section in values.yaml that allows independent configuration and the ability to disable it entirely:

headlessService:
  enabled: true  # set to false to skip creating the headless service
  annotations: {}
  additionalLabels: {}

When annotations/additionalLabels are set, they should take precedence over the corresponding service.* values for the headless service.

Read values.yaml and service-headless.yaml first to trace how the headless service currently reuses service.* values. Add the documented headlessService settings so the resource can be disabled and its annotations and labels can override service values; done means the independent configuration and disabled case are represented in the chart behavior.