Add support for Cloudflare service tokens in model requests

[Ferexx]

I have a model which is protected by Cloudflare. In order to be able to access it, I must include the "cf-access-client-id" and "cf-access-client-secret" as headers in any requests to models.

Please consider updating the schema to include support for these headers. For example:

"models": [
    {
         "provider": "some-provider",
         "baseUrl": "some-url",
         "model": "my-model",
         "token": {
             "env": "MY_SECRET",
             "cf-client-id": "MY_ID",
             "cf-client-secret": "MY_SECRET"
         }
    }
]

[brendan-kellam]

Heya - Opened up #449 for this. Lmk if you have any feedback