[request] Add Matter client/controller (subscribe) APIs to the Matter Arduino library

[hypnotoad08]

Summary

What’s the problem?

The current Arduino Matter library only implements server-side clusters and local device change callbacks. There are no APIs to:

• Discover or get a handle to a remote Matter node
• Create client clusters on that remote node
• Subscribe to attribute changes on a remote cluster
• Receive those updates via callbacks

Proposed solution

Add controller/client support alongside the existing server APIs:

1. A way to lookup/discover commissioned nodes (e.g. Matter.getDiscoveredNodeIds())
2. An API to obtain a remote-node handle (e.g. Matter.getRemoteNode(nodeId))
3. Methods to create client clusters (flags like CLUSTER_FLAG_CLIENT) on that handle
4. A subscribe(attrId) or subscribeAll() method per client cluster
5. A onAttributeReport(attrId, callback) hook for receiving updates
6. A poll() or built-in pump to drive the subscribe engine in loop()

This would enable true peer-to-peer Matter applications (e.g. an EPD dashboard subscribing to a remote AQI sensor).

References

• Espressif’s native esp_matter component provides these client APIs (esp_matter_binding_create(), esp_matter_cluster_*_client_create(), esp_matter_binding_subscribe())
• The release notes for v2.1.0 mention “device change callbacks,” but that only covers local clusters
• Example in File → Examples → Matter → matter_lightbulb_callback is purely for local attribute changes

Thanks for considering this enhancement!