Allow specifying role at runtime

[rittneje]

We would like to be able to specify the role ARN and role session name at runtime in the call to withCredentials. That way we can easily leverage multiple roles that are bound the same service user account without having to define those credentials multiple times in Jenkins. For example:

withCredentials([[$class: 'AmazonWebServicesCredentialsBinding', credentialsId: awsCredentialsId, roleArn: 'arn:aws:iam::XXXXX:role/my-role', roleSessionName: 'my-session-name']]) {
    ...
}