Support warming images by digest

[colinking]

Description

This PR adds support to the Kaniko warmer for referencing images by digest.

Before:

❯ go run ./cmd/warmer --cache-dir ./test --image=node@sha256:91ec6cb0d47628e4c1ace1efe3835aaddb2a16a9549da4f86782bb05437c53d2 --customPlatform=linux/amd64
Failed warming cache: Failed to verify image name: node@sha256:91ec6cb0d47628e4c1ace1efe3835aaddb2a16a9549da4f86782bb05437c53d2: repository can only contain the runes `abcdefghijklmnopqrstuvwxyz0123456789_-./`: node@sha256
exit status 1

After:

❯ go run ./cmd/warmer --cache-dir ./test --image=node@sha256:91ec6cb0d47628e4c1ace1efe3835aaddb2a16a9549da4f86782bb05437c53d2 --customPlatform=linux/amd64
INFO[0000] Retrieving image manifest node@sha256:91ec6cb0d47628e4c1ace1efe3835aaddb2a16a9549da4f86782bb05437c53d2
INFO[0000] Retrieving image node@sha256:91ec6cb0d47628e4c1ace1efe3835aaddb2a16a9549da4f86782bb05437c53d2 from registry index.docker.io

❯ ls -la ./test
total 680712
drwxr-xr-x   4 colin  staff        128 Apr 20 12:32 .
drwxr-xr-x  36 colin  staff       1152 Apr 20 12:32 ..
-rw-r--r--   1 colin  staff  348519424 Apr 20 12:32 sha256:91ec6cb0d47628e4c1ace1efe3835aaddb2a16a9549da4f86782bb05437c53d2
-rw-r--r--   1 colin  staff       2214 Apr 20 12:32 sha256:91ec6cb0d47628e4c1ace1efe3835aaddb2a16a9549da4f86782bb05437c53d2.json

Submitter Checklist

Not sure if this change qualifies the need for new unit tests :)

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Updates the warmer to support --image references by digest, such as --image node@sha256:91ec6cb0d47628e4c1ace1efe3835aaddb2a16a9549da4f86782bb05437c53d2