fix(types): add interface stub for RequestInit (smithy-lang#1403)

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

@@ -125,3 +125,10 @@ export interface FetchHttpHandlerOptions {
    */
   requestInit?: (httpRequest: IHttpRequest) => RequestInit;
 }
+
+declare global {
+  /**
+   * interface merging stub.
+   */
+  interface RequestInit {}
+}