Support using an assumed role ARN for cross account access

[breecoffey]

Currently, the only implemented options for cross account AWS access are using an IAM User w access key and secret id. We want to avoid that for security reasons. We should be able to specify a role ARN in the connector config that the DynamoDb connector will use.

Proposed changes:
Create an optional field in the config that is a role ARN to pass to getCredentials. If that condition is met, return an STS assume role credentials provider instead of the default credentials provider.