[Feature Request] Support deploymentRole with externalStack

[dhmw]

This is a Feature Proposal

Description

This plugin should support some sort of deploymentRole when using externalStack.

This could either use the same provider.iam.deployerRole from the main stack, or allow to be set different via custom.alerts.externalStack.deploymentRole. Either way, I would like control over this such that the external stack can be provisioned using a role other than the operator's.

Reference for how serverless itself passes deployerRole to the CloudFormation APIs:

https://github.com/serverless/serverless/blob/47752842e00b6966159deadad67b9d15d305d91b/lib/plugins/aws/lib/get-shared-stack-action-params.js#L47