Reintroduce old digest functionality

[NautiluX]

• This digest function executes HEAD requests instead of querying the manifest
• GET requests are ratelimited in some registries

Update to latest version wasn't possible due to rate limits in dependabot, see dependabot/dependabot-core#7578 (comment).

[NautiluX]

If this PR ends up being accepted, it would be awesome if you could push a tag again so we can upgrade dependabot to the latest version @deitch.

[NautiluX]

@deitch what's your thinking about bringing back the old behavior in a new function? It would allow dependabot to update to a newer version and bump images on registry.access.redhat.com without caring about rate limits for the new GET requests introduced by 1.15.
On the other hand, people relying on the new functionality in 1.15 can still make use of that function.

[deitch]

This just adds the new funcs, but doesn't change anything current?

[NautiluX]

yes exactly. It would allow us to continue using the old functionality without breaking anyone relying on the new function.

[yeikel]

Although this is a new method, see this relevant discussion #98