Support for Azure OpenAI service

[chgeuer]

• As described in https://github.com/thmsmlr/instructor_ex/pull/56, it is necessary to be able to control the full request path. The current implementation always adds /v1/chat/completions at the end, which doesn't work for Azure.
• On Azure, the Authorization: Bearer header must convey an Entra ID token, while an API key goes into the api-key HTTP header.

This commit is for the lastest version of instructor. Based on PR from @armanm, but also describe a simple EntraID token acquisition sample.