[registry] support create pull-through cache registry

[ethinx]

What

Allow k3d create pull-through cache registry

Why

for #1070

Implications

New feature, there should not be any breaking change.

k3d registry create \
  --proxy-remote-url <remote_url> \
  --proxy_username <username> \
  --proxy_password <password> \
  -v <src_path>:<dst_path> \
  ...

Demo:

1. Create a pull-through registry

k3d registry create docker-io \
  -p 127.0.0.1:5000 \
  --proxy-remote-url https://registry-1.docker.io \
  -v ~/.local/share/docker-io-registry:/var/lib/registry

2. Create registry.yaml

mirrors:
  "docker.io":
    endpoint:
      - http://k3d-docker-io:5000

3. Create cluster and use the pull-through cache

k3d cluster create cluster01 --registry-use k3d-docker-io:5000 --registry-config registry.yml

4. After the cluster01 ready, create another cluster with the same registry or rebuild the cluster, it would be fast.

k3d cluster create cluster02 --registry-use k3d-docker-io:5000 --registry-config registry.yml

[iwilltry42]

Thanks for this PR @ethinx ! 🙏
Code looks good on first glance.
Would you mind adding some documentation? It'd be good enough if you give an example containing all options in the description of this PR. In a follow-up PR one of us can add the actual documentation for k3d.io and also potentially add related options to the config file 👍

EDIT: nevermind, just copied it from your issue text 👍

[iwilltry42]

@all-contributors please add @ethinx for code

[allcontributors]

@iwilltry42

I've put up a pull request to add @ethinx! 🎉

[iwilltry42]

LGTM!
Tested and it works perfectly well 👍

[iwilltry42]

Thank you very much @ethinx ! 👍

[ethinx]

Thank you very much @ethinx ! 👍

my pleasure