Support pinning container image by digest (repo@sha256) in values

[skrawani]

It would be helpful to pin the Qdrant container image by digest for reproducible deploys. Today the chart seems to only render repository:tag.

• Expected: when a digest is provided, render repository@sha256:<digest> and prefer it over tag if both are set.
• Proposal: add image.digest to values and adjust templates to use it when present (fall back to repository:tag otherwise).

Example values:

image:
  repository: docker.io/qdrant/qdrant
  tag: v1.15.4
  digest: sha256:<digest>

Expected render:

image: docker.io/qdrant/qdrant@sha256:<digest>