Updated interface of image library with method to decode Image from string image buffer.

image.h

class ImageFactory {
public:

    // Decodes the `image_buffer` and returns Image instance.
    //
    // Suggestion for readers: Use absl::string_view instead of string here.
    static std::unique_ptr<Image> FromString(const std::string& image_buffer);
    
    // Creates an instance of 'Image' from the file descriptor 'fd'.
    //
    // Will return 'nullptr' if it's unable to decode image successfully.
    //
    // Note for readers: If you can add abseil package to your code base, I
    // recommend changing this API to return
    // 'absl::StatusOr<std::unique_ptr<Image>>' instead. This will lead to much
    // cleaner API and improved error handling.
    static std::unique_ptr<Image> FromFd(int fd);
}