Add Home Assistant integration subsystem with Zbus bridge

[Copilot]

Implements a management subsystem that bridges Zbus channels to Home Assistant, eliminating networking boilerplate for IoT device integration.

Implementation

Core subsystem (subsys/mgmt/homeassistant/)

• Background thread monitors registered Zbus channels and syncs state via Home Assistant REST API
• Supports sensors, binary sensors, switches, and numeric entities
• Configurable server address, API token, update interval, thread stack size

Public API (include/zephyr/mgmt/homeassistant/)

• HOMEASSISTANT_ENTITY_DEFINE() macro for one-line channel→entity registration
• homeassistant_register_entity() for runtime registration
• Entity types: HOMEASSISTANT_ENTITY_SENSOR, HOMEASSISTANT_ENTITY_BINARY_SENSOR, HOMEASSISTANT_ENTITY_SWITCH, HOMEASSISTANT_ENTITY_NUMBER

Sample (samples/subsys/homeassistant/basic/)

• Temperature sensor, humidity sensor, light switch demo
• Shows complete workflow from Zbus channel definition to automatic HA synchronization

Usage

// Define Zbus channel
ZBUS_CHAN_DEFINE(temperature_chan, struct temp_msg, 
                 NULL, NULL, ZBUS_OBSERVERS_EMPTY, 
                 ZBUS_MSG_INIT(.value = 20));

// Register as Home Assistant entity
HOMEASSISTANT_ENTITY_DEFINE(temp_sensor, "temperature",
                            HOMEASSISTANT_ENTITY_SENSOR,
                            "°C", "temperature", temperature_chan);

// Publish data - library handles HTTP communication
struct temp_msg temp = { .value = 25 };
zbus_chan_pub(&temperature_chan, &temp, K_NO_WAIT);

Configuration

Required prj.conf settings:

CONFIG_HOMEASSISTANT=y
CONFIG_HOMEASSISTANT_SERVER_ADDR="192.168.1.100"
CONFIG_HOMEASSISTANT_API_TOKEN="your_token"
CONFIG_ZBUS=y
CONFIG_HTTP_CLIENT=y

Limitations

• Max 16 entities (compile-time configurable)
• Simple integer data types only (extensible via callbacks)
• Unidirectional: sends state updates, doesn't receive commands
• REST API only (no MQTT discovery protocol)

Original prompt

add a new library (module?) that provides simple and easy home assistant integration

use zbus so that user needs not to think about networkin at all - they just expose data to zbus channels and boom

add a sample too

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