cli: support additional options for `files`

[pgavlin]

Some tools require that files have particular extensions in order to work properly (e.g. tf -var-file=[path] behaves differently if its argument's extension is .json). We could support situations like this by e.g. allowing the entry in files to be a dict that takes additional options:

values:
  tfConfig:
    foo: bar
  files:
    TFVARS:
      pattern: tfvars-*.json
      content: { fn::toJSON: "${tfConfig}" }

We could also allow users to set permissions, etc.