Support for invisible hCaptcha

[candidexmedia]

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

I recently learned about invisible captchas, and found out that hCaptcha supports them. Implementing it on the embedded form on my website was fairly straightforward and works beautifully! The initial implementation of hCaptcha was clunky, and didn't mesh well with the website design. This option is a lot more seamless.

Describe the solution you'd like

I would love to see an option for invisible captchas in the provided Listmonk embed code (this could be an additional checkbox in the Settings > Security captcha section, or a checkbox in the Form page).

To implement it, I had to do the following (bind the challenge to a button):

1. Add an id to the Listmonk form

2. Remove the hcaptcha divs: ```html

   <script src="https://js.hcaptcha.com/1/api.js" async defer></script> ```

3. Add an h-captcha class, and data-sitekey="your_site_key" + data-callback="onSubmit" attributes to the submit button input, like this:

   <input type="submit" value="Submit" class="h-captcha" data-sitekey="1234567" data-callback="onSubmit"/>

4. Add a data callback and the hCaptcha script:

       <script type="text/javascript">
           function onSubmit(token) {
              document.getElementById('listmonk-form').submit();
           }
        </script>
        <script src="https://js.hcaptcha.com/1/api.js" async defer></script>

The final form code looks like this:

    <form method="post" action="https://mylistmonkinstance.com/subscription/form" target="_blank" class="listmonk-form" id="listmonk-form">
        <div>
        <h3>Subscribe</h3>
        <input type="hidden" name="nonce" />
    
        <p><input type="email" name="email" required placeholder="E-mail" /></p>
        <p><input type="text" name="name" placeholder="Name (optional)" /></p>
    
        <p>
          <input id="321" type="checkbox" name="l" checked value="54321" />
          <label for="321">My List</label>
        </p>
    
    <input type="submit" value="Submit" class="h-captcha" data-sitekey="1234567" data-callback="onSubmit"/>
            
    <script type="text/javascript">
                function onSubmit(token) {
                   document.getElementById('listmonk-form').submit();
                }
    </script>
    <script src="https://js.hcaptcha.com/1/api.js" async defer></script>