Add support for passing additionalHeaders #206

We need to add support for passing custom OTLP headers through the Langfuse setup, so LibreChat can use Entra auth without forking the SDK.

Required behavior:

• allow additionalHeaders (or equivalent) to be passed into the Langfuse OTLP exporter/span processor
• preserve existing publicKey / secretKey auth
• support cookie auth headers like cookie: access_token=<token>; id_token=<token>

This lets LibreChat generate the Entra token server-side and pass it through cleanly during trace export.