Added custom Session key name setting

[mm-dsibinski]

Hi, some time ago I added a possibility to set a custom session key:

_captcha.GenerateCaptchaImageFileStream(
                _httpContextAccessor.HttpContext.Session,
                100,
                36,
                "MyCustomSessionKeyId");

It might be useful when you need to identify the captcha control is some specific or custom way.
If that's fine, I suggest merging it into the main branch. It's rather a safe change :)

[EdiWang]

Looks good, thanks!