Try using SSM parameter for IAM SSH Agent backend ARN

[keithduncan]

Allow the stack to passively discover an IAM SSH Agent backend configured using SSM.

The IAM SSH Agent ARN is needed in several places, CloudFormation parameters, dynamic task definition generation. It might make sense to make this an SSM parameter that is set once when deploying IAM SSH Agent for your infrastructure, and all the components auto discover it.

Putting the stack name in the parameter would ensure multiple deployments in the same region can use or not use iam ssh agent.