Add Thread mesh networking and border router support

[dmonagle]

Summary

Introduces comprehensive Thread mesh networking capabilities with a new border router implementation.

Changes

Package Configuration

• Added MatterThread and MatterThreadBorderRouter library targets
• Introduced trait-based conditional compilation for Thread features
• Added dependency on openthread-swift package (local path for development)

MatterThread Module

• ThreadNetworkManager: High-level API for forming, joining, and monitoring Thread networks
• ThreadInstance: Wrapper around OpenThread C APIs (assumed in openthread-swift)
• ThreadTransport: MatterUDPTransport implementation routing UDP through the Thread mesh
• ThreadDiscovery: ServiceDiscovery implementation using SRP server and mesh-local mDNS
• ThreadCommissioning: Thread-specific commissioning with dataset provisioning support

MatterThreadBorderRouter Module

• BorderRouter: Coordinates border routing services between Thread and infrastructure networks
• IPv6Routing: Manages IPv6 prefix delegation and on-mesh prefixes
• SRPProxy: Proxies Thread SRP registrations to infrastructure mDNS
• DNSProxy: Forwards DNS queries from Thread devices to infrastructure
• NAT64Translator: Enables IPv6-only Thread devices to reach IPv4 hosts

Usage Example

let manager = try ThreadNetworkManager(radioURL: "spinel+hdlc+uart:///dev/ttyACM0")
try await manager.formNetwork(name: "MyHome")

let borderRouter = BorderRouter(networkManager: manager)
try borderRouter.start()