Add support for images from docker hub

It would be cool to have support for images from docker hub.

I could imagine that to look something like this (for public images):
```yml
- task: AzureContainerAppsRC@0
inputs:
azureSubscription: 'azureSubscription'
imageToDeploy: 'docker.io/user/image:$(Build.BuildId)'
useDockerHub: true
containerAppName: 'containerAppName'
resourceGroup: 'resourceGroup'
```

Using docker hub could be implied by the `imageToDeploy` and/or the missing `acrName` and/or a `useDockerHub` property.