swagger.json validation issues

See https://online.swagger.io/validator/debug?url=https://raw.githubusercontent.com/opsgenie/opsgenie-oas/master/swagger.json

* There are a number of `nullable` properties which should probably be `x-nullable`
* Some of your `..payload` objects are defined under `#/parameters` when they should be under `#/definitions`
* Possibly other issues (see above)