feature request: Provide the docker registry service connection

Related to https://github.com/MicrosoftDocs/vsts-docs/issues/5837 and https://github.com/aws/aws-vsts-tools/issues/110. I just wanted to use the docker image on ECR to run the container job described [here](https://docs.microsoft.com/zh-cn/azure/devops/pipelines/process/container-phases?view=azure-devops&tabs=yaml). The Microsoft side suggests us to use the Docker registry service connection but it was not feasible because the token will expire in 12 hrs. So I hope the AWS extension should provide support for this by reusing some of the logic in the ECR pull task. Actually, I don't quite understand why there is no public-read ACL for the images hosted on AWS ECR.

No source file is named. Start by reading the ECR pull task logic and the linked Azure DevOps container phases documentation, then determine how the AWS extension should provide credentials for a Docker registry service connection without the 12-hour token limitation. Done means the requested ECR-backed container job can use the service connection as described.

Written by the indexing model from the issue text.