Basic Azure OpenAI Service support

[JelteF]

• Make BASE_URL configurable

Fixes #56 Fixes #57

This is different from #57 in that it uses once_cell instead of lazy_static for initialization of the default BASE_URL value.

• Add basic support for Azure OpenAI Service

Apart from choosing the BASE_URL there are some other small differences between the way you need to call the Azure service. So this adds a API_TYPE. The same approach is used in the official openai Python library.