Add forward_headers support to allow passing headers to LLM directly

[VisargD]

Add a setting named forward_headers which allows users to add header names which will be picked up from the request headers and will be directly passed to LLM request without any overwrite/manipulation from gateway. This will allow users to add custom auth headers which are not simple tokens but have multiple moving pieces like session token, user token, IAM based token etc.

This can be a part of configs as well as headers:

• forward_headers config setting which expects an array of header names.
• x-portkey-forward-headers header which accepts comma separated header names. This header will be applied globally for the request and will overwrite forward_headers sent in config for that call.