Add a status callback for MQTT subscriptions

Add a status callback for MQTT subscriptions

Integrations that use MQTT might need to wait for a subscription to complete before they initiate actions. The default behavior is that a subscription is queued and debounced, so callers usually do not wait for broker confirmation. Some integrations must guarantee that the broker finished the subscription.

The new mqtt.async_on_subscribe_done helper can be used to monitor MQTT subscriptions, to allow doing additional tasks. Make sure the same QoS is used as in the MQTT subscription.

Example:

from homeassistant.components import mqtt

async def async_setup_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
    """Setup integration MQTT subscription monitoring."""

    def _on_subscribe_status() -> None:
        """Handle subscription ready signal."""
        # Do stuff

    # Handle subscription ready status update
    await mqtt.async_on_subscribe_done(
        hass,
        "myintegration/status",
        qos=1,
        on_subscribe_status=_on_subscribe_status,
    )

    # Do stuff