feat(anvil.js): Add support for specifying headers in fork mode

.changeset/odd-bears-relax.md

@@ -0,0 +1,5 @@
+---
+"@viem/anvil": patch
+---
+
+Added support for specifying headers in fork mode

packages/anvil.js/src/anvil/createAnvil.ts

@@ -105,6 +105,14 @@ export type AnvilOptions = {
    * on disk, anything missing locally would be fetched from the remote.
    */
   forkChainId?: number | undefined;
+  /**
+   * Specify headers to send along with any request to the remote JSON-RPC server in forking mode.
+   * 
+   * e.g. "User-Agent: test-agent"
+   * 
+   * Requires `forkUrl` to be set.
+   */
+  forkHeader?: Record<string, string> | undefined;
   /**
    * Initial retry backoff on encountering errors.
    */

packages/anvil.js/src/anvil/toArgs.ts

@@ -7,13 +7,27 @@ import { toFlagCase } from "./toFlagCase.js";
  * @returns The command line arguments.
  */
 export function toArgs(options: {
-  [key: string]: string | boolean | number | bigint | undefined;
+  [key: string]: Record<string, string> | string | boolean | number | bigint | undefined;
 }) {
   return Object.entries(options).flatMap(([key, value]) => {
     if (value === undefined) {
       return [];
     }
 
+    if (typeof value === "object" && value !== null) {
+      return Object.entries(value).flatMap(([subKey, subValue]) => {
+        if (subValue === undefined) {
+          return [];
+        }
+
+        const flag = toFlagCase(key);
+
+        const value = `${subKey}: ${subValue}`;
+
+        return [flag, value];
+      });
+    }
+
     const flag = toFlagCase(key);
 
     if (value === false) {