digesta-bot: Support Helm Chart (separate image and tag fields)

Lots of helm charts have `image:` and `tag:` on separate lines. For example from SPIRE helm chart:
```
    registry: cgr.dev
    # -- The repository within the registry
    repository: chainguard/bash
    # -- Overrides the image tag
    tag: latest@....
```
Would be great if digesta bot handled this layout.
