feature suggestion: option to create a deployment IAM user

[bvalosek]

Great plugin... I find myself using it in nearly all of my sls projects now.

One thing I frequently do when using this plugin is to create an IAM user in the Resources section of my serverless.yml that has the minimally required permissions in order to run sls deploys in a CI/CD environment that strictly create a change set. This makes it easy then to create API keys for that user and have CI/CD use those creds to deploy cfn changesets automatically.

Would having an optional flag/config in this plugin that would automatically add that user and permissions to the cloudformation template make sense? This would remove having to copy/paste the same IAM user/permissions around, and this plugin can have a nice way of asserting the permissions it needs to create the changesets.

If this doesnt feel too niche or specific / out-of-scope, then I could start a PR.