Add MAC address to device info

[kahrendt]

• Auto-detects primary interface on ESP32
• Attempts to make a best guess on host builds
• Overridable by the user config
• Makes all device information fields optional in SendspinClientconfig (breaking change for anyone who reads from the struct)

[Copilot]

Pull request overview

This PR extends the client/hello device identity payload to optionally include a MAC address, with platform-specific auto-detection (ESP-IDF default netif, host best-effort “primary” routable interface) and a user override via SendspinClientConfig. It also makes the other device identity fields optional in the public config struct and updates tests/docs accordingly.

Changes:

• Add device_info.mac_address (optional) to protocol structs and JSON serialization.
• Introduce platform_get_interface_mac() with ESP-IDF and host implementations, and wire it into SendspinClient::build_hello_message() (config override + auto-detect fallback).
• Add unit tests for optional device_info field emission and host MAC detection contract; update docs and build files.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_protocol.cpp	Adds coverage ensuring optional device_info fields (including mac_address) serialize only when present.
tests/test_network_info.cpp	Adds host-side contract test for MAC auto-detection (absent or well-formed lowercase MAC).
tests/CMakeLists.txt	Includes the new network-info unit test in the aggregated test binary.
src/protocol.cpp	Serializes device_info.mac_address when set.
src/protocol_messages.h	Extends DeviceInfoObject with optional mac_address.
src/platform/network_info.h	Introduces platform-abstracted platform_get_interface_mac() API and documents behavior.
src/host/network_info.cpp	Implements host MAC detection via getifaddrs() with a “routable interface” heuristic.
src/esp/network_info.cpp	Implements ESP-IDF MAC detection via default netif MAC, with efuse STA MAC fallback.
src/client.cpp	Populates hello device_info.mac_address from config override or platform detection.
include/sendspin/config.h	Makes device identity fields optional and adds optional mac_address with documented semantics.
docs/integration-guide.md	Updates integration docs for optional identity fields and documents mac_address.
CMakeLists.txt	Adds esp_netif to ESP-IDF component requirements to support ESP MAC detection.
cmake/sources.cmake	Adds the new ESP/host network_info.cpp sources to the per-platform source lists.

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