Accept `user-data` as a json object

_This issue was originally opened by @curiositycasualty as hashicorp/packer#9235. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

#### Feature Description
The amazon-* builders only accept `user-data` as a string. I'd much prefer if packer accepted a json object and rendered it as a string for AWS's sake OR explicitly accepted a "cloud_init_user_data" param that would render as yaml (including the mandatory first "#cloud-config" line). Do so would force JSON compliance on the contents of the param and aid in `cloud-init` configuration.