Support custom image downloads and digests to be specified as configuration settings

There are use-cases when images need to be fetched from custom locations instead of github.

The images and their digest used by colima are currently hard-coded.
https://github.com/abiosoft/colima/blob/ac876a5d9e4eda7d1d015896ebfbe1dd16b8c203/environment/vm/lima/yaml.go#L20-L21

Overriding the images with `~/.lima/_config/override.yaml ` as in https://github.com/abiosoft/colima/issues/121 does not work reliably, since the hard-coded images are used as a fallback in case when the image override fails.