feat: add support for custom serviceAccount name for lake and ui deployments

[3khenrique]

What changes were introduced?

• Added optional serviceAccount.name parameter to lake and ui sections in values.yaml
• Updated lake and ui deployment templates to conditionally set serviceAccountName when a value is provided

Why is this change needed?

Currently, the Helm chart offers no way to assign a custom ServiceAccount to the lake or ui pods. They always fall back to the namespace default. This is a common requirement in environments with fine-grained RBAC, Workload Identity (GCP) or other pod-level identity mechanisms.

Key features:

• Backward compatible: fields default to empty. Existing installations are unaffected.
• Non-invasive: serviceAccountName is only injected into the pod spec when a name is explicitly provided

Usage example:

lake:
  serviceAccount:
    name: "devlake-lake-sa"

ui:
  serviceAccount:
    name: "devlake-ui-sa"

[klesh]

LGTM.
Thanks for your contribution.