fix(helm): Support exisiting ServiceAccount

[ChinmayPande48]

Summary

This PR adds support for using an existing Kubernetes ServiceAccount with the Helm chart.
Fixes: (#459)

Changes

• Added a conditional guard around templates/serviceaccount.yaml so a ServiceAccount is only created when serviceAccount.create is true.
• Updated the StatefulSet to use the existing qdrant.serviceAccountName helper instead of hardcoding the ServiceAccount name.
• Added serviceAccount.create and serviceAccount.name to the default values.yaml to expose the configuration already supported by the helper.

With these changes, users can set:

serviceAccount:
  create: false
  name: existing-service-account

to reuse a pre-existing ServiceAccount, while preserving the current default behavior (create: true) for existing deployments.