Add streaming response support

[tw4]

Summary

Nexar does not support streaming HTTP responses. This limits its use in scenarios like downloading large files, reading Server-Sent Events (SSE), or consuming chunked transfer responses.

Use Cases

• Downloading large files without loading the entire body into memory
• Consuming SSE streams (e.g., AI model token streaming)
• Real-time data feeds with chunked transfer encoding

Proposed Solution

Add dedicated streaming methods that return a Stream or expose an IAsyncEnumerable<string> for line-by-line reading.

// Stream download
await using var stream = await Nexar.GetStreamAsync(url);

// SSE / line-by-line
await foreach (var line in Nexar.GetLinesAsync(url))
{
    Console.WriteLine(line);
}