text: Title: Support YAML OpenAPI Specs in Addition to JSON
  Body: **Is your feature request related to a problem? Please describe.**
  
  We maintain our OpenAPI spec in YAML, which is supported by many spec validators and other OpenAPI tooling. It would be great to able to generate the client directly from it instead of relying on conversion.
  
  **Describe the solution you'd like**
  Be able to directly import YAML like
  
  ```bash
  openapi-python-client generate --path openapi.yaml
  ```
  
  **Describe alternatives you've considered**
  
  Currently during CI, we're exporting YAML to JSON like:
  
  ```bash
  yq -j -P r openapi.yaml > openapi.json
  ```
  
  However, this is potentially unsafe since YAML is a superset of JSON and supports some features.
  