Add transport for low-level network connection

[mattt]

This PR adds a NetworkTransport implementation to the MCP Swift SDK, providing network-based communication capabilities alongside the existing StdioTransport.

Features

• Implements the Transport protocol using Apple's Network framework
• Handles connection establishment, message sending and receiving over network connections
• Properly manages connection state with async/await patterns
• Includes robust error handling and logging
• Works with any NWConnection instance, allowing flexibility in network configuration

Example Usage

import Network
import MCP

// Create a TCP connection to a server
let connection = NWConnection(
    host: "example.com", 
    port: 8080, 
    using: .tcp
)

// Create the transport with the connection
let transport = NetworkTransport(connection: connection)

// Use with MCP client
let client = Client(name: "MyApp", version: "1.0.0")
try await client.connect(transport: transport)

Model Context Protocol currently supports only stdio and SSE interfaces. However, NetworkTransport can be helpful when proxying traffic from stdio over a network interface.