Fetch containerd auth config from the credentials file

[kron4eg]

What this PR does / why we need it:
Instead of storing registry authentication info directly in the kubeone config, it's now possible to have this info fetched from the credentials file.

The API reference ContainerRuntimeContainerd and it's used together with apiVersion and kind fields for type metadata.

Limitation: ONLY ContainerdRegistry.Auth will be extracted from credentials file.

Example:

registriesAuth: |
  apiVersion: kubeone.k8c.io/v1beta2
  kind: ContainerRuntimeContainerd
  registries:
    auth:
      docker.io:
        username: "u5er"
        password: "myc00lp455w0rd"
        auth: "base64(user:password)"
        identityToken: ""  

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1740

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Fetch containerd auth config from the credentials file

[kubermatic-bot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[xmudrii]

Just one question, otherwise LGTM.

[xmudrii]

/lgtm
/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 94c75369acf972abba6bfd374d599c67ab73c2fb

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kron4eg, xmudrii

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