fix: oras-go client should fallback to docker config if no credentials specified

[tonyay163]

I'm using #17279 to authenticate to Google Artifact Registry as a helm registry using workload identity. Unlike #10218, the other solution does not require installing ESO. This works in general since helm template can get creds via the $HOME/.docker/config.json. However, if you need to use targetRevision: * with your application, this ends up using the oras-go client here which is only configured for static credentials.

This change adds a backwards compatible change where if both the username and password is not configured, it will use the oras-go credentials package to get the docker config from the environment and use that to get credentials.

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[tonyay163]

I tested this ETE and it seems to work well