add generate images-sync-config command

[tashima42]

Adds the release generate rancher images-sync-config command, which generates a regsync config file, used to sync images from one registry to another.

Example usage:

export DOCKERIO_REGISTRY_USERNAME=username
export DOCKERIO_REGISTRY_PASSWORD=password
export PRIME_REGISTRY_USERNAME=username
export PRIME_REGISTRY_PASSWORD=password

release generate rancher images-sync-config -k "tashima42/rancher:v2.9.0,tashima42/rancher-agent:v2.9.0" -s docker.io -t registry.rancher.com -o ./config.yaml

regsync check --config ./config.yaml
regsync once --config ./config.yaml

[briandowns]

Didn't mean to approve. Meant to request changes.

approved on accident