API Gateway - SSM api keys

[fernando-mf]

This is a Bug Report

Description

Currently it is not possible to use api keys from SSM. Normally this syntax works for lambdas

apiKeys:
  - name:  myApiKey
    value: ${ssm:/apiKey}

But when we use it for step functions the following error is thrown

API Keys must be strings

It would be nice to add support for ssm keys since they are preserved after stack removals so no need to update the keys in the client when the stack is fully restarted