[Feature Request] Support an OpenAPI generator plugin

[ttc0419]

Feature description:
OpenAPI is a well known RESTful API documentation format, plugins like https://github.com/kollalabs/protoc-gen-openapi has partial support for protoc-gen-validate. With protovalidate reached 1.0, please consider adding validation rule generator to tools like https://github.com/google/gnostic/tree/main/cmd/protoc-gen-openapi.