Support YAML OpenAPI Specs in Addition to JSON

[bowenwr]

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

openapi-python-client generate --path openapi.yaml

Describe alternatives you've considered

Currently during CI, we're exporting YAML to JSON like:

yq -j -P r openapi.yaml > openapi.json

However, this is potentially unsafe since YAML is a superset of JSON and supports some features.

[bowenwr]

Awesome, thank you!