Turn multi-SFU media transport into a developer option

src/state/Async.ts

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+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import {
+  catchError,
+  from,
+  map,
+  Observable,
+  of,
+  startWith,
+  switchMap,
+} from "rxjs";
+
+export type Async<A> =
+  | { state: "loading" }
+  | { state: "error"; value: Error }
+  | { state: "ready"; value: A };
+
+export const loading: Async<never> = { state: "loading" };
+export function error(value: Error): Async<never> {
+  return { state: "error", value };
+}
+export function ready<A>(value: A): Async<A> {
+  return { state: "ready", value };
+}
+
+export function async<A>(promise: Promise<A>): Observable<Async<A>> {
+  return from(promise).pipe(
+    map(ready),
+    startWith(loading),
+    catchError((e) => of(error(e))),
+  );
+}
+
+export function mapAsync<A, B>(
+  async: Async<A>,
+  project: (value: A) => B,
+): Async<B> {
+  return async.state === "ready" ? ready(project(async.value)) : async;
+}