Support repository regex rewrite rules when fetching image content.

Example configuration:
```yaml
# /etc/rancher/k3s/registries.yaml
mirrors:
  "docker.io":
    endpoint:
    - "https://registry-1.docker.io/v2"
    rewrite:
      "^library/alpine$": "my-org/alpine"
```

This will instruct k3s containerd to fetch content for `alpine` images
from `docker.io/my-org/alpine` instead of the default
`docker.io/library/alpine` locations.
Signed-off-by: Jacob Blain Christen <jacob@rancher.com>