Add the netif BSD sockets index (necessary for running mDNS on the ESP-IDF)

[ivmarkov]

With the big stream of changes in #233 / #237 - and particularly the Networks' management - I did not carry over the network interface index (a STD / BSD sockets' thing) from rs-matter-stack into rs-matter as rs-matter and the Network Diagnostics cluster impl in particular does not need it as it is not reported it in the cluster attributes.

However I forgot that the built-in mDNS responder might also use the NetifInfo type and the NetifDiag trait downstream (and it does!).

The problem is specifically when rs-matter is running on top of ESP-IDF (with the esp-idf-matter helper crate) because - for some reason - ESP-IDF really does not like to open a multicast UDP socket with the "default"/"all" network interface = 0. It must be the index of a concrete network interface (Eth or Wifi), as ESP-IDF cannot multicast on "all".

So when the built-in mDNS responder grabs the most suitable netif reported by NetifDiag on which to operate, it does it with interface index 0, and opening the mDNS multicast socket immediately fails on the ESP IDF therefore.

The solution is to "thread" the interface index together with the other netif parameters in the NetifInfo struct, so that the built-in mDNS can grab the correct index for that particular netif.

[gemini-code-assist]

Summary of Changes

Hello @ivmarkov, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a necessary change to the NetifInfo structure to include the network interface index. This is crucial for ensuring compatibility and correct behavior of the built-in mDNS responder, particularly when running on platforms like ESP-IDF that require explicit interface binding for multicast operations.

Highlights

• Network Interface Information: Added a new field netif_index of type u32 to the NetifInfo struct in rs-matter/src/data_model/sdm/gen_diag.rs. This field is intended to store the BSD sockets index for the network interface.
• Platform Compatibility (ESP-IDF): The addition of the netif_index is specifically required to address an issue on the ESP-IDF platform where the built-in mDNS responder needs a concrete network interface index (not 0) to successfully bind multicast UDP sockets.

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[gemini-code-assist]

Code Review

This pull request introduces a netif_index field to the NetifInfo struct. This change is well-motivated by the need to support mDNS on ESP-IDF, which requires a specific network interface index for multicast operations.

The primary feedback focuses on enhancing the robustness of this new field by suggesting the use of Option<u32> instead of u32. This change would make it explicit when an interface index is not available, aligning with Rust best practices and helping to prevent potential issues arising from ambiguous interpretations of a default value like 0.

[bjoernQ]

LGTM