Add support for YAML Spec Plugins (#2054)
It's common to use `yaml` for an OpenAPI spec used in the GPT plugins.
For example: https://www.joinmilo.com/openapi.yaml or
https://api.slack.com/specs/openapi/ai-plugin.yaml (from [Wong2's
ChatGPT Plugins List](https://github.com/wong2/chatgpt-plugins))
