Add headers option to custom LLM models

Author: bajajcodes

api.ts

@@ -4801,6 +4801,10 @@ export interface CustomLLMModel {
   metadataSendMode?: "off" | "variable" | "destructured";
   /** These is the URL we'll use for the OpenAI client's `baseURL`. Ex. https://openrouter.ai/api/v1 */
   url: string;
+  /**
+   * Custom headers to send with requests to the custom LLM endpoint. These headers can override default OpenAI headers except Authorization.
+   */
+  headers?: Record<string, string>;
   /**
    * This sets the timeout for the connection to the custom provider without needing to stream any tokens back. Default is 20 seconds.
    * @min 20