docs: update corporate-proxy.mdx with https-proxy-agent example (#12817)

Update corporate-proxy.mdx

Added HttpsProxyAgent support to corporate proxies. Undici library fails behind certain firewalls and on edge runtime.

Author: serolfy

docs/pages/guides/corporate-proxy.mdx

@@ -8,6 +8,8 @@ Auth.js libraries use the `fetch` API to communicate with OAuth providers. If yo
 
 You can provide a custom `fetch` function by passing it as an option to the provider.
 
+# Using Undici Library
+
 Here, we use the `undici` library to make requests through a proxy server, by passing a `dispatcher` to the `fetch` implementation by `undici`.
 
 <Code>
@@ -32,6 +34,35 @@ export const { handlers, auth } = NextAuth({
   </Code.Next>
 </Code>
 
+# Using HttpsProxyAgent
+
+On Edge Runtimes or with proxy restrictions,  the `undici` library may not work. Using a simpler approach with HttpsProxyAgent by passing a `proxyAgent` to the `fetch` implementation.
+
+<Code>
+  <Code.Next>
+
+```tsx filename="auth.ts"
+import NextAuth, { customFetch } from "next-auth"
+import GitHub from "next-auth/providers/github"
+const { HttpsProxyAgent } = require('https-proxy-agent');
+
+const proxyAgent = new HttpsProxyAgent("my.proxy.server");
+async function proxy(url: string, options: any): Promise<Response> {
+  const response = (await fetch(url, {
+    ...options,
+    agent: proxyAgent,
+  })) as unknown as Response;
+  return response;
+}
+
+export const { handlers, auth } = NextAuth({
+  providers: [GitHub({ [customFetch]: proxy })],
+})
+```
+
+  </Code.Next>
+</Code>
+
 ## Resources
 
 - [`undici` - Basic Proxy Request with local agent dispatcher](https://undici.nodejs.org/#/docs/api/ProxyAgent?id=example-basic-proxy-request-with-local-agent-dispatcher)