feat: add a passthrough map to configuration

[jeacott1]

It would be helpful for downstream configuration if kubestack users could include arbitrary per environment configuration in the standard structure that was passed through to the output.

ie: given the usual

configuration = {
    apps = {
       #add a map where users can just pass through whatever they like.
       custom = {
          ENV_mycustomconfig = ""
          DISK_size = 500
       }
    }

This could be exported as its own map(map(string/any/?)) and subsequently used in arbitrary foreach blocks instead of having to duplicate the configuration/apps/ops/whatever to add custom bits and pieces. I suggest a separate export here just because 3 levels of map usually gets awkward in tf.

thoughts?