[ember] relax ObserverMethod property name arg (#28839)

* [ember] failing test - relax ObserverMethod property name arg

* [ember] relax ObserverMethod property name arg

types/ember/index.d.ts

@@ -143,7 +143,7 @@ declare module 'ember' {
 
     type ObserverMethod<Target, Sender> =
         | (keyof Target)
-        | ((this: Target, sender: Sender, key: keyof Sender, value: any, rev: number) => void);
+        | ((this: Target, sender: Sender, key: string, value: any, rev: number) => void);
 
     interface RenderOptions {
         into?: string;

types/ember__utils/ember__utils-tests.ts

@@ -8,6 +8,7 @@ import {
     tryInvoke,
     typeOf
 } from '@ember/utils';
+import EmberObject from '@ember/object';
 
 (function() {
     /** isNone */
@@ -161,3 +162,10 @@ import {
     isEmpty({ size: 1 }); // $ExpectType boolean
     isEmpty({ size: () => 0 }); // $ExpectType boolean
 })();
+
+class Foo extends EmberObject.extend({
+    abc: true,
+    bar() { return '123'; }
+}) {
+    def: 'hello';
+}