Add generic MqttClient trait with multi-backend support

[MathiasKoch]

Summary

Introduces a generic MqttClient trait that decouples all rustot services from concrete MQTT implementations. Four backend implementations are provided: embedded-mqtt (no_std/embassy), rumqttc (std/tokio), AWS Greengrass IPC, and a mock client for testing. All services — shadows, OTA, provisioning, defender metrics, and jobs — are migrated to accept any MqttClient implementation via generics.

This enables the same application code to run across bare-metal embedded devices, Linux gateways, and Greengrass components by swapping the MQTT backend at compile time.

Note: This is PR 2 of 4 in a stacked series. Depends on #82 (KV-based shadow persistence).

Design

MqttClient Trait (src/mqtt/mod.rs)

pub trait MqttClient {
    type Subscription<'m, const N: usize>: MqttSubscription where Self: 'm;
    type Error: Debug;

    fn client_id(&self) -> &str;
    async fn publish<P: ToPayload>(&self, topic: &str, payload: P) -> Result<(), Self::Error>;
    async fn subscribe<const N: usize>(&self, topics: &[(&str, QoS); N]) -> Result<Self::Subscription<'_, N>, Self::Error>;
}

Key design decisions:

• &self methods — Interior mutability allows sharing across async tasks without &mut exclusivity
• Const generic topic count — subscribe<const N: usize> enables zero-cost static dispatch
• ToPayload / DeferredPayload — Lazy serialization directly into the transmit buffer, avoiding intermediate allocations
• Associated error type — Each backend surfaces its native error type

Backend Implementations

Backend	Feature flag	Environment	File
embedded-mqtt	embedded_mqtt	no_std / embassy	src/mqtt/embedded.rs
rumqttc	rumqttc	std / tokio	src/mqtt/rumqttc.rs
Greengrass IPC	greengrass	std / tokio	src/mqtt/greengrass.rs
Mock	(always available)	testing	src/mqtt/mock.rs

• rumqttc: Spawns background EventLoop poller, routes messages to subscriptions via mpsc channels with MQTT wildcard matching (+/#)
• Greengrass: Merges per-topic native subscriptions via futures::stream::SelectAll
• embedded-mqtt: Zero-copy, direct trait mapping with PayloadBridge adapter

Service Migration

All services now accept C: MqttClient as a generic parameter instead of depending on a specific MQTT implementation. The Mqtt<C> newtype wrapper provides service-specific trait implementations.

Changelog

• Add MqttClient, MqttSubscription, MqttMessage, and ToPayload traits in src/mqtt/
• Add embedded-mqtt, rumqttc, greengrass, and mock backend implementations
• Migrate shadow cloud service to generic MqttClient
• Migrate OTA control/data interfaces to generic MqttClient
• Migrate provisioning service to generic MqttClient
• Migrate defender metrics service to generic MqttClient
• Replace ShadowTestOnly with MockMqttClient in shadow tests
• Fix Greengrass multi-topic subscription using SelectAll
• Loosen 'static requirement on topic string references