Make version optional for OCI charts for promotion workflows

### What would you like to be added?

By using OCI images, versions are generally expressed by tags. The actual image content doesn't need to have version information contained. This allows images to be promoted by retagging.

The author of an helm chart should be able **not specifing** `version:` in `Chart.yaml`. At the moment a chart is pulled / installed from the OCI registry it infers the version from the tag being used to pull.

For compatibliy reasons helm could write the `version` on pulling, so subsequent operations like install / upgrade can rely on the version to be present.

### Why is this needed?

A common release / gitops pattern is promotion. From a ref in code, like a tag an image is build. Once it passes necessary approvals / tests the image is just retagged, but not rebuild. This ensures signatures are still valid, and it is the same artifact.

Currently I need to build a whole new image, even if just the version changes.