feat: make ADC the default option for GCP authentication when using go-containerregistry

[renzodavid9]

Related with:

• Use gcloud for authenticating to AR's *-docker.pkg.dev #7063
• Decouple Skaffold from gcloud CLI #9457

Description
This is to remove the dependency with gcloud CLI when interacting with a private Google Registry using the go-containerregistry libray. Changing the implementation of Skaffold's Keychain to first check if it can get the access token through Application Default Credentials, if not possible then it will fallback to use gcloud CLI.

I tried to use the google.Keychain authenticator, but it will first check if the registry is a known Google registry, if not it will fail. This will be breaking a case where users have a custom domain pointing to a Google Registry and configured in their .docker/config.json:

{
  "credHelpers": {
    "mydomain.com": "gcloud" # <- mydomain.com pointing to AR
  },
}

Looks like that case is possible according to https://cloud.google.com/blog/topics/developers-practitioners/hack-your-own-custom-domains-container-registry

Also extending logic to detect known Google Registries (so they don't need to be configured in .docker/config.json)

Follow-up Work (remove if N/A)
We're still missing more places to remove the gcloud dependency. More coming.