Add API entity to backend catalog-info.yaml. similar to https://github.com/janus-idp/backstage-plugins/blob/main/plugins/ocm-backend/catalog-info.yaml#L29-L61 

This way, the backend plugin OpenAPI definition would be readable and usable from our internal dogfooding instance, like with the OCM Plugin:

Actually, this can be applied to all backend plugins exposing an API.