Add POST methods with raw binary data and ContentReceiver support for streaming responses

[Copilot]

This PR implements the feature request from Issue #2210, adding support for Client POST requests with raw binary data (const char *body, size_t content_length) and ContentReceiver for streaming responses.

Problem

The existing httplib library supported ContentReceiver (for streaming responses) with POST requests, but only for std::string body data. There was no way to send raw binary data with a specific length while also receiving streaming responses. This limitation prevented use cases like:

• LLM REST servers that accept binary payloads and stream token responses
• Server-Sent Events (SSE) from POST endpoints with binary data
• Consistent API usage between string and binary data for streaming responses

Solution

Added 4 new POST method overloads to both Client and ClientImpl classes:

// Without custom headers
Result Post(const std::string &path, const char *body, size_t content_length, 
           const std::string &content_type, ContentReceiver content_receiver, 
           DownloadProgress progress = nullptr);

// With custom headers  
Result Post(const std::string &path, const Headers &headers, const char *body, 
           size_t content_length, const std::string &content_type, 
           ContentReceiver content_receiver, DownloadProgress progress = nullptr);

Implementation Details

• ClientImpl methods: Create Request objects and use req.body.assign(body, content_length) to properly handle binary data including null bytes
• Client methods: Delegate to corresponding ClientImpl methods following established patterns
• Consistent API: Uses the same ContentReceiver pattern as existing string-based methods
• Binary data support: Correctly handles data containing null bytes via explicit length parameter

Example Usage

httplib::Client cli("llm-server.com", 8080);
const char* binary_payload = "..."; // Binary data
size_t payload_size = 1024;

// Stream response while sending binary data
auto res = cli.Post("/v1/chat/completions", binary_payload, payload_size, 
                   "application/octet-stream",
                   [](const char* data, size_t len) {
                       std::cout << std::string(data, len); // Stream tokens
                       return true;
                   });

Testing

Added comprehensive test cases covering:

• POST with raw binary data and ContentReceiver
• POST with custom headers, raw binary data and ContentReceiver
• Binary data containing null bytes
• Verification that existing methods remain functional

Backward Compatibility

This change is fully backward compatible - no existing API signatures were modified, only new overloads were added.

Fixes #2210

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• httpbin.org
  • Triggering command: ./test_issue_2210 (dns block)

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• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
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