roleArn on AWSShellScript

In many cases I use AWSShellScript to execute aws cli commands, but I haven't seen a nice solution on how to run an AWSShellScript with an assumed role other than the serviceconnection role

Just like the CloudFormationCreateOrUpdateStack take a roleArn parameter and pass it to CloudFormation it would be nice if AWSShellScript could do the same, take a roleArn parameter and assume the role and execute the cli command

Right now i need to script my way out of it. (or am i missing something ?)

```
temp_role=$(aws sts assume-role --role-arn "arn:aws:iam::my_account:role/myrole" --role-session-name "MySession")
export AWS_ACCESS_KEY_ID=$(echo $temp_role | jq -r .Credentials.AccessKeyId)
export AWS_SECRET_ACCESS_KEY=$(echo $temp_role | jq -r .Credentials.SecretAccessKey)
export AWS_SESSION_TOKEN=$(echo $temp_role | jq -r .Credentials.SessionToken)
aws s3 ls my-bucket
```

Start at the AWSShellScript task entry point and compare how CloudFormationCreateOrUpdateStack accepts and uses roleArn. Verify the requested roleArn behavior and confirm that an AWSShellScript command runs with the assumed role, adding or updating relevant tests if the task exposes them.

Written by the indexing model from the issue text.