Jsonnet native support

[zhulik]

https://jsonnet.org is a configuration language/json templater. In practice it's very convenient for creating complex json documents such as k8s manifests or OpenAPI definitions.

Currently in order to use it with oapi-codegen one needs to compile a jsonnet document into yaml or json and then call the generator.

Even though the original implementation of jsonnet is in c++, there is an official port in go https://github.com/google/go-jsonnet and it's quiet simple to use.

Here I'm asking whether the authors of this project and the community are interested in adding native support for jsonnet to the generator and volunteering myself to implement it.