Support custom reCAPTCHA providers

[z38]

Support hCaptcha and other API-compatible reCAPTCHA providers by adding options to set custom endpoints.

Example:

recaptcha:
  version: 2-checkbox
  theme: light
  site_key: 10000000-ffff-ffff-ffff-000000000001
  secret_key: '0x0000000000000000000000000000000000000000'
  js_url: 'https://hcaptcha.com/1/api.js'
  verify_url: 'https://hcaptcha.com/siteverify'

Fixes #423

[NicoHood]

I like the idea! Can we also add this information as an example to the readme? So people will know about the hcaptcha alternative, without looking through all PRs (which nobody will do)

Edit: and once the pr got merged you can add a PR here as well:
https://github.com/hCaptcha/hcaptcha-integrations-list

[z38]

@NicoHood I thought about showing the integration of hCaptcha on https://learn.getgrav.org/17/forms/forms/fields-available#captcha-field. As for the integration list, will do!