Add uploadLargeFormFile() for memory-efficient multipart uploads

[prestonbrown]

The problem

uploadFormFile() loads the whole file into memory before sending. That's fine for small files, but when you're uploading 50-100MB files on an embedded device with 512MB RAM... yeah, not great.

I hit this while building a 3D printer touchscreen UI. Uploading large G-code files was causing the whole system to choke.

What this adds

New function uploadLargeFormFile() - like uploadFormFile() but streams the file in chunks (same pattern as uploadLargeFile() vs uploadFile()).

// Basic usage (filename derived from path)
auto resp = requests::uploadLargeFormFile(
    "http://server/upload",
    "file",                    // form field name
    "/path/to/huge_file.bin",
    params,                    // extra form fields
    [](size_t sent, size_t total) {
        printf("%.1f%%\n", 100.0 * sent / total);
    }
);

// With custom upload filename (when local path differs from desired name)
auto resp = requests::uploadLargeFormFile(
    "http://server/upload",
    "file",
    "/tmp/modified_version.bin",
    "original_name.bin",       // upload filename override
    params,
    progress_cb
);

Memory usage for a 100MB file:

• uploadFormFile(): ~100MB
• uploadLargeFormFile(): ~40KB (just the chunk buffer)

Implementation

I extracted a shared helper to avoid duplicating the streaming loop from uploadLargeFile():

• New: internal::streamFileToConnection() helper - extracted from uploadLargeFile()
• Refactor: uploadLargeFile() now uses the shared helper (same behavior, no breaking change)
• New: uploadLargeFormFile() for multipart uploads, also uses the shared helper

No breaking changes - existing code works exactly as before.