Feature Request: Support for custom Redis clients (Upstash, Valkey, etc.)

Problem

Currently, the library only supports two specific Redis client libraries:

• redis (via resumable-stream)
• ioredis (via resumable-stream/ioredis)

Users who want to use other Redis-compatible clients like Upstash Redis, Valkey, can do so but they are still required to install one of the above libraries, even though they are unused

Use Cases

1. Upstash Redis: Users deploying to edge environments (Cloudflare Workers, Vercel Edge Functions) often use Upstash Redis REST API
2. Valkey: Users migrating from Redis to Valkey (Redis fork) need compatibility
3. Custom implementations: Teams with custom Redis wrappers or proxies can't integrate easily
4. Future-proofing: New Redis-compatible clients emerge regularly

Proposed Solution

Add a generic entry point that accepts custom Publisher/Subscriber implementations conforming to the existing interfaces:

import { createResumableStreamContext } from "resumable-stream/generic";
import type { Publisher, Subscriber } from "resumable-stream/generic";

// Users provide their own implementations
const streamContext = createResumableStreamContext({
  waitUntil: null,
  publisher: myCustomPublisher,
  subscriber: myCustomSubscriber,
});

Compare the existing resumable-stream and resumable-stream/ioredis entry points and the Publisher and Subscriber interfaces described in the issue. Trace how the current Redis clients are wired, then define the generic entry point so callers can provide custom implementations; done means the shown import and configuration work without requiring either existing client library.