Add support for the ReCaptcha onload callback

[mroskamp]

Is your feature request related to a problem? Please describe.

I use JQuery Unobtrusive Validation to validate my forms. I also use the ReCaptcha feature to protect my forms against robots. I'd like to be able to add validation to the reCAPTCHA form element, which is loaded dynamically and thus can't be added until after the reCAPTCHA is loaded. Fortunately, Google provides an onload parameter that you can provide on the ReCaptcha script (see documentation). Unfortunately, there isn't a convenient way to specify that parameter and when using the ReCaptchaTagHelper.

Describe the solution you'd like

Add an attribute to the ReCaptchaTagHelper for specifying the onload callback.

Describe alternatives you've considered

For my site, I replaced the base Orchard Core ReCaptchaTagHelper with a custom implementation with the onload attribute.

[hishamco]

Can you create a PR for it?