Make the AzureContainerInstanceJob creds block optional for AzureContainerInstanceCredentials infra block.

[taylor-curran]

As you can see in the screen shot, a credentials block is required.

Many users want to configure credentials at the resource level rather than creating a credential and entrusting it to Prefect. For example, we commonly see that Azure KeyVault is used to store secrets and users prefer to only have their secrets stored in one place, so enabling these blocks to function without storing secret values in Prefect is important.