feat(medusa-react,medusa-js): Allow custom headers (#4409)

* add support for passing custom headers to backend through medusa-js and medusa-react

* switch order of override of headers

* Create gentle-teachers-greet.md

---------

Co-authored-by: Oliver Windall Juhl <59018053+olivermrbl@users.noreply.github.com>

packages/medusa-js/src/request.ts

@@ -16,6 +16,7 @@ export interface Config {
   maxRetries: number
   apiKey?: string
   publishableApiKey?: string
+  customHeaders?: Record<string, any>
 }
 
 export interface RequestOptions {
@@ -199,6 +200,9 @@ class Client {
     options: RequestOptions = {},
     customHeaders: Record<string, any> = {}
   ): Promise<any> {
+   
+   customHeaders = { ...this.config.customHeaders, ...customHeaders }
+
     const reqOpts = {
       method,
       withCredentials: true,