Blueprints: support array input and methods

I would like my Blueprints to support adding arbitrary environment variables to the stacks it creates.

Example usage:

inputs:
  - id: env_vars
    name: Extra env vars, entered as key=val
    type: array
[...]
stacks:
  - key: mystack
    environment:
      variables:
        - name: TF_VAR_name
          value: mystack
        ${{ -- some way to add input.env_vars -- }}

Example usage would be to pass parameters like env_vars=”key1=foo,key2=foo”