Openapi kt

[nomisRev]

This PR replaces the OpenAPI Generator using Mustache, with a Kotlin based generator. Which has support for a much wider range of complex operations, and types, and currently fully supports the OpenAI specification, and all OpenAPI types.

The changes in the PR are minimal:

• nested schemas are now generated as nested models so CreateChatCompletionModel becomes CreateChatCompletion.Model.
• There is no longer a difference between model, and api packages, simplifying imports of the generated code
• Downloading the OpenAI OpenAPI Specification is refactored to Gradle Kotlin

A couple fixes need to be back ported still:

• firstId and lastId should become non-required.
• Serialisation Exception should include Response Info for logging