support YAML format for the specification in OpenAPI framework

[stevehu]

When we adding the OpenAPI framework late last year, the design was influenced by the Swagger framework which can only support JSON format for the specification as there are limitations in the swagger.io libraries. In the OpenAPI framework, we had implemented our own openapi-parser as there was no existing parser at the time. The library does support both JSON and YAML for the specification but the handler class set the expectation of JSON format.

With some requests from the community and the multiple chain support introduced. It is time to support both YAML and JSON. The /health and /server/info won't be injected to the specification anymore in the future and the user can just copy the specification to the config folder once the spec is updated and chances are most users will use YAML format instead.

After the update, the OpenAPI framework will look up the specification file in the following sequence.

• openapi.yml
• openapi.yaml
• openapi.json

If openapi.yml exists, it won't look up others anymore. If openapi.yaml exists, it won't look up the .json anymore.