fix(types): add interface stub for RequestInit

Author: kuhe

.changeset/forty-cooks-deny.md

@@ -0,0 +1,5 @@
+---
+"@smithy/types": patch
+---
+
+add interface stub for browser RequestInit type

packages/types/src/http/httpHandlerInitialization.ts

@@ -123,5 +123,12 @@ export interface FetchHttpHandlerOptions {
    * });
    * ```
    */
-  requestInit?: (httpRequest: IHttpRequest) => RequestInit;
+  requestInit?: (httpRequest: IHttpRequest) => {} extends RequestInit ? never : RequestInit;
+}
+
+declare global {
+  /**
+   * interface merging stub.
+   */
+  interface RequestInit {}
 }