host-ctr: use user-provided reg creds for 'public.ecr.aws'

[etungsten]

Issue number:
Resolves #2671

Description of changes:

    host-ctr: use user-provided reg creds for 'public.ecr.aws'
    
    If users provide their own registry credential for 'public.ecr.aws' then
    use that credential instead of the fetched credentials from ECR public.

Testing done:

Validated by @ahreehong and @vignesh-goutham.

To summarize:
On vmware-k8s-1.23, specified a private registry mirror for public.ecr.aws and specified registry creds for the registry mirror.

[settings.container-registry.mirrors]
"public.ecr.aws" = ["https://198.18.34.118:443"]
[settings.pki.registry-mirror-ca]
data = "<redacted>"
trusted=true
[[settings.container-registry.credentials]]
registry = "public.ecr.aws"
username = "<redacted>"
password = "<redacted>"

host-ctr is able to pull images from the registry mirror at 198.18.34.118 and start host containers and bootstrap containers.

There is a separate issue we discovered as part of this validation. cri-containerd requires the registry creds to be set for the registry mirror itself (198.18.34.118) and not the destination registry host (public.ecr.aws in this case). There needs to be a follow-up fix to align host-ctr's way of constructing the image resolver with cri-containerd's. See #2677 for more info.

This PR is still valid on it's own regardless; Users might still want to be able to get public ECR creds themselves and pass it manually through settings.container-registry.credentials for public.ecr.aws.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[stmcginnis]

Looks good!

[jpmcb]

Beautiful! 👏🏼