Add support for custom headers

[sterankin]

I don't see a way to be able to add a custom header (or headers) to a request. It would be great if the client code was change such that the headers could be added to.

We are required in our use cases to provide a user header but this library won't allow clients to pass this (unless I am mistaken).

https://github.com/andrewyng/aisuite/blob/main/aisuite/providers/azure_provider.py#L31