Separate ECR account block/allow from AWS auto-auth

[squaremo]

It's possible to want to exclude (or include) ECR accounts from scanning, even if you aren't using the AWS authentication for whatever reason.

I think it'd be fairly easy to always respect the block/allow arguments, either by having two ImageCreds... decorators, or by adapting the ImageCredsWithAWSAuth so the auto-auth bit is optional.

[kingdonb]

No further enhancements can land in Flux v1,

• Flux v1 is in maintenance mode #3320