Add Ability to Reference Existing Roles

[sam-mundle]

The title pretty much says it all. Would be great because we maintain all of our roles in a separate system and we want to keep our serverless.yml as short as possible

[MozartLino]

I think you don't need this plugin, You can use the role property to define an existing role:

Ex:

some:
  handler: index.someHandler
  role: arn:aws:iam::xxxxxx:role/role-that-already-exists

for more information: https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-resource-function.html#sam-function-role