ci: move public-api goldens to goldens directory (#35768)

Moves the public api .d.ts files from tools/public_api_guard to
goldens/public-api.

Additionally, provides a README in the goldens directory and a script
assist in testing the current state of the repo against the goldens as
well as a command for accepting all changes to the goldens in a single
command.

PR Close #35768

goldens/public-api/http/testing/testing.d.ts

@@ -0,0 +1,21 @@
+/** @deprecated */
+export declare class MockBackend implements ConnectionBackend {
+    connections: any;
+    connectionsArray: MockConnection[];
+    pendingConnections: any;
+    constructor();
+    createConnection(req: Request): MockConnection;
+    resolveAllConnections(): void;
+    verifyNoPendingRequests(): void;
+}
+
+/** @deprecated */
+export declare class MockConnection implements Connection {
+    readyState: ReadyState;
+    request: Request;
+    response: ReplaySubject<Response>;
+    constructor(req: Request);
+    mockDownload(res: Response): void;
+    mockError(err?: Error): void;
+    mockRespond(res: Response): void;
+}