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/common/http/testing/testing.d.ts

@@ -0,0 +1,43 @@
+export declare class HttpClientTestingModule {
+}
+
+export declare abstract class HttpTestingController {
+    abstract expectNone(url: string, description?: string): void;
+    abstract expectNone(params: RequestMatch, description?: string): void;
+    abstract expectNone(matchFn: ((req: HttpRequest<any>) => boolean), description?: string): void;
+    abstract expectNone(match: string | RequestMatch | ((req: HttpRequest<any>) => boolean), description?: string): void;
+    abstract expectOne(url: string, description?: string): TestRequest;
+    abstract expectOne(params: RequestMatch, description?: string): TestRequest;
+    abstract expectOne(matchFn: ((req: HttpRequest<any>) => boolean), description?: string): TestRequest;
+    abstract expectOne(match: string | RequestMatch | ((req: HttpRequest<any>) => boolean), description?: string): TestRequest;
+    abstract match(match: string | RequestMatch | ((req: HttpRequest<any>) => boolean)): TestRequest[];
+    abstract verify(opts?: {
+        ignoreCancelled?: boolean;
+    }): void;
+}
+
+export declare interface RequestMatch {
+    method?: string;
+    url?: string;
+}
+
+export declare class TestRequest {
+    get cancelled(): boolean;
+    request: HttpRequest<any>;
+    constructor(request: HttpRequest<any>, observer: Observer<HttpEvent<any>>);
+    error(error: ErrorEvent, opts?: {
+        headers?: HttpHeaders | {
+            [name: string]: string | string[];
+        };
+        status?: number;
+        statusText?: string;
+    }): void;
+    event(event: HttpEvent<any>): void;
+    flush(body: ArrayBuffer | Blob | string | number | Object | (string | number | Object | null)[] | null, opts?: {
+        headers?: HttpHeaders | {
+            [name: string]: string | string[];
+        };
+        status?: number;
+        statusText?: string;
+    }): void;
+}