Update openai client generation

[fedefernandez]

Some context:

• All the classes in the package com.xebia.functional.openai.models.ext. are manually created
• All the classes in the package com.xebia.functional.openai but not in models.ext are generated automatically from the API

This PR:

• Delegates the model classes generation to the generation library (removing the manual ones)
• Updates the API template for exposing the HTTP client. The file ApiClient.kt.mustache is a copy from the original template with the change in the lateinit var client: HttpClient (removing the private modifier). This is reflected in ApiClient.kt
• Adds a new generic type, OpenAIModel, to customize the model sent (it can be one of the supported model by the specific endpoint or a custom model)
• Updates de KMMGeneratorConfig.java to support the generation of anyOf models

[Montagon]

Great job, @fedefernandez! 🙌