[FEATURE] Support apiKeyHelper-equivalent for custom headers

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When connecting to Claude Enterprise through a model gateway/proxy, it's often necessary to supply two different authentication keys: one for the model gateway, and another for Claude.ai. A common approach is to use a custom header to specify the gateway key (as is suggested by LiteLLM), so that the ordinary authentication fields can be used for Claude Enterprise credentials.

If the gateway key is ephemeral, this introduces a challenge: there's no refresh mechanism for custom headers. apiKeyHelper is sufficient when only a gateway key is required (and can be passed through the ordinary authentication fields), but insufficient when two keys are required.

Proposed Solution

There should be a apiKeyHelper equivalent for custom headers. This would allow the ephemeral gateway token to be dynamically injected into requests.

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

Configuration and settings

Use Case Example

Example scenario: User is interacting with Claude.ai through an AI gateway, and authenticates to both Claude.ai and the AI gateway as themselves. The AI gateway token is ephemeral, and needs to be updated without burdening the user to take manual action on token expiration.

Additional Context

_No response_