Add multi-proxy support & granular proxy selection

[CorentinB]

Implements a granular multi-proxy system with fine-grained control over proxy selection.

Features

• Multi-proxy support: Configure multiple proxies with automatic round-robin selection
• Network filtering: Filter proxies by IPv4/IPv6 support via ProxyNetwork enum
• Type classification: Categorize proxies as Mobile/Residential/Datacenter via ProxyType enum
• Domain routing: Route specific domains to specific proxies using glob patterns
• Per-proxy statistics: Track RequestCount, ErrorCount, and LastUsed per proxy
• Context-based selection: External callers can request specific proxy types via context

Example: Context-based proxy type selection

// Configure proxies with different types
httpClient, _ := warc.NewWARCWritingHTTPClient(warc.HTTPClientSettings{
    Proxies: []warc.ProxyConfig{
        {
            URL:     "socks5://mobile-proxy:1080",
            Network: warc.ProxyNetworkAny,
            Type:    warc.ProxyTypeMobile,
        },
        {
            URL:     "socks5://datacenter-proxy:1080",
            Network: warc.ProxyNetworkAny,
            Type:    warc.ProxyTypeAny,
        },
    },
})

// Request specific proxy type via context
ctx := warc.WithProxyType(req.Context(), warc.ProxyTypeMobile)
req = req.WithContext(ctx)
resp, _ := httpClient.Do(req)

Breaking change

Replaces Proxy string field with Proxies []ProxyConfig in HTTPClientSettings.