feat: can use existing Redis client in the watcher

[Copilot]

Users with restricted access to connection details or centralized Redis clients cannot reuse existing client instances when creating watchers. The SubClient and PubClient fields in WatcherOptions were typed as *rds.Client, preventing use with cluster clients.

Changes

• Type change: SubClient and PubClient in WatcherOptions now accept rds.UniversalClient instead of *rds.Client

  • Enables passing any client type: regular, cluster, sentinel, failover, ring
  • Backward compatible: *rds.Client implements rds.UniversalClient

• Documentation: Added examples for using existing clients with both regular and cluster modes

Usage

// Create your own cluster client
clusterClient := redis.NewClusterClient(&redis.ClusterOptions{
    Addrs: []string{"localhost:7000", "localhost:7001", "localhost:7002"},
})

// Pass it to the watcher (empty addrs ignored when clients provided)
w, _ := rediswatcher.NewWatcherWithCluster("", rediswatcher.WatcherOptions{
    SubClient: clusterClient,
    PubClient: clusterClient,
    Channel:   "/casbin",
})

Works identically with NewWatcher() for regular clients.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>No option to init new watcher (clustered) with redis client</issue_title>
<issue_description>Currently, there is no way to initialize a new watcher (in clustered mode) using an existing Redis client instance. The code expects Redis connection details like host, port, or options to internally create a new Redis client.

In my use case, I already have an active Redis client (due to restricted access to connection details or centralized Redis setup), and I need to reuse this client to create a clustered watcher.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes No option to init new watcher (clustered) with redis client #51

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