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.

[willmhowes]

Love to see the golang-style enum pattern being used here!

[willmhowes]

Why continue without error if URL is blank?

[willmhowes]

To me, it seems that ProxyTypeAny represents something different than the other ProxyTypes because it describes the traffic's preference, where as the other types describe the kind of infrastructure the proxy uses
I think part of my confusion is that I understand you might want to send a URL to a proxy with residential IPs, or you might want to send a URL to a proxy with datacenter IPs, but ProxyTypeAny isn't a kind of proxy but is actually a decision that you don't care which proxy type you send the data to
So I'm confused under what condition you would define a proxy as ProxyTypeAny?

[willmhowes]

I didn't finish submitting my review back when this was fresh, so concluded my thoughts and pushed the review today. That being said, perhaps this issue should be closed given the discussion in #186, specifically #186 (comment). @NGTmeaty ?